hydro_scotty
I ran across some information on growing red leaf lettuce about a year or so ago, but I haven't been able to locate it again yet. I don't know for sure, but if I remember correctly the key to getting the red color in the lettuce was in the nutrient solution. That is a specific mineral element that the plant needs to produce the color. Though I don't even want to try and guess. I'll keep looking for it, but if you contact a few nutrient manufactures and ask them, you may get the right answer, or at least point you in the right direction.

I just Wanted to clarify a little more. I know your nutrient solution probably contains the mineral element that the plant needs to produce the color, but needs more to get the rich coloring your looking for. It could also be reducing the the amount of some mineral element/s in the nutrient solution that aid in the production of chlorophyll in the plants leaves will do the same thing. Whichever it is I don't really want to guess. As I mentioned, I would contact many nutrient manufactures. Your bound to reach someone familiar with growing red leaf lettuce, or at least know someone who does like a commercial grower they sell products to.

Thanks homehydro. I noticed a couple of years ago that when I let my nutrients deplete to certain levels, my red head popped but it was just not manageable for all the other plants. I think you're right but it is hard to find help on this. If you find it, please let me know. Thanks

Try reducing the magnesium and see if that helps. Insufficient magnesium can lead to the reddening of leaves in green-leaved plants; it might be that this species is red-leafed because it requires more magnesium than most plants do to maintain green chlorophyll production. If you mix your own nutrients, it might be easiest to use half the amount of epsom salts you're currently feeding it.

Granted, I could be entirely wrong, but there shouldn't be much harm in testing this for a week or two.

hydro_scotty
No problem, I'll keep looking when I have time. But by your response I think you'll need to grow the red leaf lettuce in a separate system than your other plants. I'm not sure if it was an article I read, or a video I saw. But it was about/from a commercial grower, and said how they got the deep redness. I'm sure they were only growing red leaf lettuce in that system though. If your growing it with other plants, there will need to be a compromise in one crop or the other/s.

Thanks everyone. I'll give some things a try and try to get back on here and let you know the results.

I have been growing some red lettuce indoors and it was staying very green. after taking them outside a couple times I noticed every time they were exposed to a little bit of cold they got redder. might be a temperature thing as lettuce is a cool weather plant

I can attest with what youngngreen said. I also noticed that with warmer surroundings, the red pigment fades fast. I used the same nutrient for both btw.

Maye its a way of the lettuce saying its stressed