Fairly clearly black-and-white is not what you want; “colour” is.
The real answer is that there is no antonym of greyscale, just as there is no antonym of chalk or cheese or grass or green. Greyscale does not form half of any pair, let alone of a pair of opposites.
If this is a purely linguistic question that’s all there is to it.
If this is not a linguistic question but one of technical vocabulary, which we might as well call jargon, the useful answer is as suməlic said: It depends a lot on what you mean precisely and what the context is.
If, for instance, you were trying to categorize colours used in a program into greyscale or (answer here) colours, the closest you could get to an acronym would describe the maximum range of colours used in that particular program, which should be “full colour” but sounds as though it isn’t.
If that was the case, we might expect the program’s own requirements or limitations or design parameters to define the answer before the question was asked.
Nevertheless I use the word “range” to make it clear that I am talking not about the full “spectrum” of all possible colour, nor about a lesser “gamut”, being the reduced spectrum used by any given device, whether it be a printer, scanner, screen or what have you. For a program to use anything other than the full spectrum or the gamut of a particular device would be an arbitrary choice but perhaps one justifiable for technical reasons.
A cursory glance at comments and answers even here, reveals a broad spectrum of opinion about the whole idea much of which, while perfectly sensible, is not technical terminology but logical extrapolation from ordinary English.
Printers acknowledge broadly three and a half colour levels: Mono, which is only just what it says on the tin; Greyscale or simply grey, which comes in various denominations; Colour, which need not be full colour; Full colour, which in an ideal world would be and in much software is the entire spectrum but in the real mechanical world is restricted to the physical gamut of any given device.
In the context of categorizing colours into greyscale and otherwise there is no antonym because greyscale does not form half of any pair, let alone of a pair of opposites. All the various denominations of greyscale sit between mono and full colour. That technically, greyscale is a subset of all colour does not linguistically make colour an antonym of greyscale, nor anything like it
Mono is only just what it says on the tin because it refers to one ink, but please, how when one ink is applied to any paper, could the result be a single colour? Neither solid black nor solid white is by any means similar to “black and white”, however convenient the term “mono” might seem.
Greyscale, or simply grey, comes in various denominations. What is common or even technically possible today has never been relevant to any theory either of colour or of grammar.
In any language the technicalities are wholly irrelevant; nevertheless, some early monitors displayed only 4-bit colour, equating to 16 shades of grey; some used 8 bits for 256 shades; 16 bits representing 65,536 shades gives largely pointless distinction in greyscale, very restrictive for “full” colour today but once a great leap forward.
Largely because today’s “true” colour has 24 bits, giving 8 to each red, green or blue channel, “monochromatic” has evolved to encompass the 256 levels available in any single, 8-bit channel. Normal usage makes that channel grey, not red or green or blue but it still needs to be recognized that “monochromatic” actually means not “one colour”, but “one colour” of ink.
Some systems change matters as much linguistically as technically by using 30 or 36 or even 48 bits to show billions of colours and even those are sometimes modified to use 40 or 48 or even 64 bits.