Can you post a photo?

Could you elaborate on your situation? Are these plants indoors or outdoors? How long since they were moved to where they are? Is this covered in another thread, and if so, could you link to it?

I remember your post. If cutting back on the fert is definitely what you think caused the yellowing, i'd move back to whatever fertilization plan you had them on before. Unfortunately, once some veggie plants get used to a lot of fert every 2 weeks or whatever, they get dependant upon it and will wilt and die if you cut it back.

Very very new to this but I think that purple tomato leaves are a lack of potassium. At least that is what my Gardener's Bible says.

The purple is caused from a lack of potassium. Fertilizing should help. When they are in the ground epsom salt helps.

Leaves on tomato turning purple very often indicate that they are simply too cold. Toms do not like even close to chilly temps.
Yellow leaves- fertilize them and water them as much as you think they need- not soaping wet but nice steady moisture.
I have done full strength fertilizer from the day they sent out true leaf and nothing bad happened. Have done that for years.

Being cold and a little purple probably won't kill the tomatoes, but not watering might. Mine start to yellow like you described when I let them get too dry. I start my tomatoes under lights in the basement, where it's often a little chillier than is ideal, and the undersides of their leaves are often a little purple. No big deal.