Sounds like a nutient deficient soil - if you can get your hands on a soil-testing kit, do it as it will tell you what your soil lacks in terms of Nitrogen, Phosphorous or Potassium (N-P-K).

I had a similar problem a few years ago, newly transplanted tomato plants started to get sickly grey-looking leaves. The soil test revealed a deficiency in Nitrogen. I quickly added Blood meal to correct the problem.

Yellowing of leaves can be a lack of nitrogen, but since it is nice new mix maybe the problem is some chemical in the soil or else, maybe the compost part of the mix has something like manure that wasn't properly composted/broken down and is too strong for the plants? Just guessing here. Does the soil have a very fresh manure smell to it?

Sounds like nitrogen deficiency. The topsoil and sand may have none, and the compost may not have enough. Especially if it's old compost made from largely woody material.

I would suggest a shot of soluble fertilizer, such as Miracle Gro. See if it greens them up. If it does, you might want to continue fertilizing and/or use organic fertilizers like alfalfa meal regularly.

Thanks for all of your help. My first thought was that it was a nitrogen deficiency. We have dug in some mushroom compost and have used a liquid fertilizer. I will let you know if it worked. Thanks again