The leaves of my young tomato plants or going yellow/white, what is wrong ?

it could be they are to cold , where are you keeping them?

Agree with Mac, also it could be mildew growing in a low temperature. Start again.

Started seeds in kitchen then transfered them to greenhouse.

Is the greenhouse heated?, and were they in direct sunlight in the greenhouse, with water on the leaves.

Greenhouse is not heated, they were in the direct sunlight,not sure if there was water on the leaves

This could be scortch, as the sun magnifies the water on the leaves. When putting small seedlings in your greenhouse again, cover with a sheet of newspaper. Also at night if it is frosty.