I feed mine all the time, however the food is very diluted, even more so than they may suggest on the package. I have noticed that my streps prefer natural sunlight but I still have many of mine under growlights. I also have some of the violet barn hybrids. The majority of my streps have bright green leaves. Could it be that the strep was unhealthy so the leaves weren't exactly the right color and now they are coming back correctly and you think it could be wrong? What is the hybrid's name?
Buying store made mixes is tough. I would not recommend ANY if you wick water. If you wick water I would recommend buying soil from a violet greenhouse (like lyndon lyon or violet barn)
You can find commercially available african violet soil (if you do not wick water), which would be ok but it is a bit heavier than most people like. The best soil mix (and easy to make!) is 1 part vermiculite, 1 part peat moss and 1 part (or a little less)of perlite. Get the coursest vermiculite you can buy.
-renee

I wish I knew NinaM. My inclination is that the plant is not taking up enough nutrients like iron. It doesn't matter that it is getting the same treatment as others.

I have a batch of babies from the same leaf and one plantlet has been chartreuse while the others are a healthy green. It seems to be greening up now though -- several months later.