When we talk about colour, can we say 'it is in pink'? "In' in the sentences is necessary? OR it has different meaning?
Yes, you can speak about colors that way, but only in certain contexts, where a verb plus in would be relevant to the object being described. For example, we’re familiar with the idea of letters being set in a font, so an expression like the following sounds natural:
Joe: What did you think of the code editor?
Moe: I liked it. The keywords were in pink, and the globals in a kind of dark teal. Not your usual scheme at all.
Other verbs that you can use include painted, illuminated, dressed and so on, and they don’t have to be present explicitly so long as the reader can infer their presence.
Coach (to players): We’ll do a scrimmage. Yesterday’s starters in black. Everyone else in pink.
Travel reporter: They did one of those French light and sound things. Greek buildings in blue. Romans in red. Anything after the 1300’s they just left black.