There are no difficulties in telling the colour once you've got the rgb triplet. Use a lookup table. Or convert it to hsv, where h looks up from the scale red-orange-yellow-green-blue-purple or something similar. And v adds the words light or dark, if needed. Just add more words, if you want. And deal with the s value too.
The difficulty is getting the rgb value in the first place. You won't get a single thread colour to a simple colour detector. Either you take a photo of a single thread and use whatever magnifying lense, or you use your three cell colour sensor and read the colour from the reel, where you have tens or hundreds of turns of the thread to get a colour surface.