Actually, it doesn't tell you the "best color to use" ... in the sense of "what color the fish want to bite". It just tells you what colors are most visible at the depth of the probe.
I had one of the original Color-C-lector units ... and in all the waters I fished at that time, it mostly told me to fish blue/char, which was a color I was already using & catching fish on. I sold that one and later on bought the Combo-C-lector ... but never fully utilized it, and it eventually ended up in the dark confines of my "fishing equipment closet". The main reason was that it was (in my mind) just too time consuming to use.
I've always meant to take it along & "test" it ... not using it to tell me what colors to use, but to use the colors I choose (first) then when I catch fish on them ... check the unit to see if it agrees. I've just never remembered to include it in my equipment arsenal.