Even though it's made for seedlings -- light, airy & drains well -- it still needs nitrogen.

nitrogen, mine were in pots and I overwatered them, I reduced the water, added some nitro and voila nice and dark green again.! Thanks to Dave and others who are nice enough to help a brother out!

Thanks for the tips! I found another posting on the same question after I wrote this -- funny, I did a search for "yellow cucumber leaves" before I posted, and nothing came up, but when I perused the postings later, I found a post with almost that exactly as the title!