(1) *The word highlighted redly caught her eye.
(2) The word highlighted in red caught her eye.
(3) The word highlighted red caught her eye.

(1) is ungrammatical, (2) is grammatical and idiomatic, and (3) is grammatical albeit less idiomatic than (2). Now, redly is unequivocally an adverb and (2)'s red is a noun. But I'd say (3)'s red is more like an adjective describing the color attribute of word than an adverb describing the manner in which it is highlighted.

Here's an attested example of (3):

As part of the new technique, the reader's eye is drawn to the 'optimal recognition point' in the word, which is highlighted red.

<2018 The Telegraph>

Based on this observation, we can say (4) doesn't work but (5) does:

(4) *The redly highlighted word caught her eye.
(5) The red highlighted word caught her eye.

Here are attested examples (Boldface mine):

She looked up at the red highlighted letters hanging over the entrance: Wild Clover Seed Message.

<Marriage in the Red>

Vineet typed in something in the FIND command function window and the text scrolled to a yellow highlighted word. The word was "nuke."

<Ballistic>

When you are done sharing your screen, return to the Zoom app and tap the red Stop Share button, or tap on the upper-left corner of your screen on the red-highlighted clock feature from anywhere else you may be browsing or sharing on your phone. In other words, you do not have to actively click back to the app to stop sharing your screen if you do not wish to. NOTE: "Red-highlighted clock" refers to the typical clock telling you the time, seen on the upper left of your phone (unless otherwise set up to appear elsewhere). When the clock is highlighted in red, it means your screen is being recorded and, in the case of Zoom, shared. This is the same red highlight that would appear over your clock if you were to use the Record Screen function outside of Zoom.

<Introduction to Teaching with Zoom>

Even in jeans and a simple top, Noriko exuded femininity with her careful makeup and red-highlighted hair that fell in soft layers around her face.

<2002 The New York Times>

And in all these red is an adjective.

(1) *The word highlighted redly caught her eye.
(2) The word highlighted in red caught her eye.
(3) The word highlighted red caught her eye.

(1) is ungrammatical, (2) is grammatical and idiomatic, and (3) is grammatical albeit less idiomatic than (2). Now, redly is unequivocally an adverb and (2)'s red is a noun. But I'd say (3)'s red is more like an adjective describing the color attribute of word than an adverb describing the manner in which it is highlighted.

Here's an attested example of (3):

As part of the new technique, the reader's eye is drawn to the 'optimal recognition point' in the word, which is highlighted red.

<2018 The Telegraph>

Based on this observation, we can say (4) doesn't work but (5) does:

(4) *The redly highlighted word caught her eye.
(5) The red highlighted word caught her eye.

Here are attested examples (Boldface mine):

She looked up at the red highlighted letters hanging over the entrance: Wild Clover Seed Message.

<Marriage in the Red>

Vineet typed in something in the FIND command function window and the text scrolled to a yellow highlighted word. The word was "nuke."

<Ballistic>

When you are done sharing your screen, return to the Zoom app and tap the red Stop Share button, or tap on the upper-left corner of your screen on the red-highlighted clock feature from anywhere else you may be browsing or sharing on your phone. In other words, you do not have to actively click back to the app to stop sharing your screen if you do not wish to. NOTE: "Red-highlighted clock" refers to the typical clock telling you the time, seen on the upper left of your phone (unless otherwise set up to appear elsewhere). When the clock is highlighted in red, it means your screen is being recorded and, in the case of Zoom, shared. This is the same red highlight that would appear over your clock if you were to use the Record Screen function outside of Zoom.

<Introduction to Teaching with Zoom>

Even in jeans and a simple top, Noriko exuded femininity with her careful makeup and red-highlighted hair that fell in soft layers around her face.

<2002 The New York Times>

“Don’t be afraid to fight,” shouted the director, Alex Dmitriev, on the third go around of a scene between the young characters Catherine and Rodolpho. “This is a lover’s quarrel.” Scripts with yellow highlighted lines were strewn about. Props wouldn’t be brought in until tech week around May 23, so actors were improvising with preschool furniture and toys.

<2018 The New York Times>

And in all these red is an adjective.

(1) *The word highlighted redly caught her eye.
(2) The word highlighted in red caught her eye.
(3) The word highlighted red caught her eye.

(1) is ungrammatical, (2) is grammatical and idiomatic, and (3) is grammatical albeit less idiomatic than (2). Now, redly is unequivocally an adverb and (2)'s red is a noun. But I'd say (3)'s red is more like an adjective describing the color attribute of word than an adverb describing the manner in which it is highlighted.

Here's an attested example of (3):

As part of the new technique, the reader's eye is drawn to the 'optimal recognition point' in the word, which is highlighted red.

<2018 The Telegraph>

Based on this observation, we can say (4) doesn't work but (5) does:

(4) *The redly highlighted word caught her eye.
(5) The red highlighted word caught her eye.

Here are attested examples (Boldface mine):

She looked up at the red highlighted letters hanging over the entrance: Wild Clover Seed Message.

<Marriage in the Red>

When you are done sharing your screen, return to the Zoom app and tap the red Stop Share button, or tap on the upper-left corner of your screen on the red-highlighted clock feature from anywhere else you may be browsing or sharing on your phone. In other words, you do not have to actively click back to the app to stop sharing your screen if you do not wish to. NOTE: "Red-highlighted clock" refers to the typical clock telling you the time, seen on the upper left of your phone (unless otherwise set up to appear elsewhere). When the clock is highlighted in red, it means your screen is being recorded and, in the case of Zoom, shared. This is the same red highlight that would appear over your clock if you were to use the Record Screen function outside of Zoom.

<Introduction to Teaching with Zoom>

Even in jeans and a simple top, Noriko exuded femininity with her careful makeup and red-highlighted hair that fell in soft layers around her face.

<2002 The New York Times>

And in all these red is an adjective.

(1) *The word highlighted redly caught her eye.
(2) The word highlighted in red caught her eye.
(3) The word highlighted red caught her eye.

(1) is ungrammatical, (2) is grammatical and idiomatic, and (3) is grammatical albeit less idiomatic than (2). Now, redly is unequivocally an adverb and (2)'s red is a noun. But I'd say (3)'s red is more like an adjective describing the color attribute of word than an adverb describing the manner in which it is highlighted.

Here's an attested example of (3):

As part of the new technique, the reader's eye is drawn to the 'optimal recognition point' in the word, which is highlighted red.

<2018 The Telegraph>

Based on this observation, we can say (4) doesn't work but (5) does:

(4) *The redly highlighted word caught her eye.
(5) The red highlighted word caught her eye.

Here are attested examples (Boldface mine):

She looked up at the red highlighted letters hanging over the entrance: Wild Clover Seed Message.

<Marriage in the Red>

Vineet typed in something in the FIND command function window and the text scrolled to a yellow highlighted word. The word was "nuke."

<Ballistic>

When you are done sharing your screen, return to the Zoom app and tap the red Stop Share button, or tap on the upper-left corner of your screen on the red-highlighted clock feature from anywhere else you may be browsing or sharing on your phone. In other words, you do not have to actively click back to the app to stop sharing your screen if you do not wish to. NOTE: "Red-highlighted clock" refers to the typical clock telling you the time, seen on the upper left of your phone (unless otherwise set up to appear elsewhere). When the clock is highlighted in red, it means your screen is being recorded and, in the case of Zoom, shared. This is the same red highlight that would appear over your clock if you were to use the Record Screen function outside of Zoom.

<Introduction to Teaching with Zoom>

Even in jeans and a simple top, Noriko exuded femininity with her careful makeup and red-highlighted hair that fell in soft layers around her face.

<2002 The New York Times>

And in all these red is an adjective.