Johan_Ha:
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.
I missed the part about the reel (bobbin?) but it's nice to know that you agree with my earlier posts on the rest.