At this early stage it is easy to jump to conclusions. It may be a quick question but may not be a quick answer.

The problem could be cultural like too much water, not enough sun, not enough water, poor drainage, wrong ambient temperature, etc. Were they outside during the recent cold snap? Those things can be changed without too much trouble.

Have you grown tomatoes in the same location before? Is this one plant or several?

This could also be caused by a nutrient deficiency. Yellow with dark spots and small, narrow leaves could be lack of zinc. Yellow with green veins could be lack of iron and yellow with green veins that are turning bronze could be a potassium deficiency.

Then there is the dreaded (at least to me) Fusarium Wilt or Verticillium wilt. This could come from previous soil contamination or it could have come from bad transplants. They never seem to improve and become stunted.

Lets hope it is not that, but one way to know is to cut open the stems. If they are discolored inside, you would need to destroy the plant (not in the compost pile) and start again with a tomato that is resistant AND IN NEW SOIL!

It is probably too early for insects but check to see if the leaves have a sticky feel to them. If so, check for insects under the leaves.

There are other diseases and root issues, but we need more information. Check with the local extension service for when someone is going to be there and bring them a few leaves to examine and describe your micro-environment.