I have a lilac bush that was near access ground water and the leaves had yellow in them. I read online the yellow leaves could be caused by too much water. it may be different for tomato plants though. This year, we've changed some things. Im curious to see how much better the lilac bush will do without all the water going so close to it. Not sure about the pepper leaf....
Are you planting the tomato plants and the peppers in the same place as they were planted before? Both are VERY hungry plants and can easily deplete the soil that they are in. My recommendation is to get an organic fertilizer such as Nutri Rich or something similar along with some good compost and dig it in around your plants and water well. Mulch the plants again quite deeply with compost.
The yellowing in the leaves might also be an iron deficiency in which case a dose or two of chelated iron wouldn't hurt.
Confusingly, darkness on pepper plant leaves can be due to either over or under watering, or fungal infection. Rotate the crops and try not to plant the same stuff in the same place each year unless you do a really good deep cover crop/compost enrichment.
Did you fix your leaf problem?
Was it wet and cool at the time of your post? Cool wet weather can cause leaf problems like your having.
If you were to fertilize your plants us a good brand that contains nutrients from a number of ingredients.
I myself like Down to Earth bio-alive, it contains:
NITROGEN TOTAL (N) 5.0%
0.4% Water Soluble Nitrogen
4.6% Water Insoluble Nitrogen