When the leaves of your veg plants turn yellow between the veins you can safely assume that they are deficient in magnesium or another vital nutrient. Don’t panic though, you can restore them to good health quite quickly with a good quality liquid feed.
The most common nutrients for a veg plant to be deficient in are nitrogen, magnesium and potassium, so make sure that the feed has plenty of these in it. As a general rule, tomatoes need plenty of potassium, so tomato feeds are usually high in potassium.
The other thing to look at if your plants have unhealthy looking leaves is the amount of water that is in the soil. Too much waterlogging can actually make it difficult for the plant to take up the nutrients, and equally if the soil is too dry it is difficult for them as well. So you need to try and make sure that they have good quality, moist but not waterlogged soil.
Once you’ve checked the water levels and added a liquid feed, don’t expect your plant to bounce back overnight. The affected leaves will not recover, but any new leaves that appear should be a nice, healthy shade of green. It’s a good idea to cut off any badly affected leaves, just so that it’s easier to track any improvements, or lack thereof.