If ‘greening up’ your lettuce is the goal, make sure you’re fertilizing with a higher Nitrogen fertilizer. Lettuces love Nitrogen as much as my mule, Ithaca, loves carrots! :)
“Nitrogen imbalance: Lettuce requires a balanced supply of nutrients, especially nitrogen. Too much nitrogen can result in excessive leaf growth but may delay or inhibit the development of heads. On the other hand, a nitrogen deficiency can lead to stunted growth and yellowing leaves. It’s important to provide a balanced fertilizer or amend the soil with organic matter to maintain the proper nitrogen levels.”
https://thegardenmagazine.com/best-fertilizer-for-lettuce-when-to-feed-your-lettuce/
I have plenty of “well cured” chicken guano I can add. :-)
(You once castigated me for adding too much of the above around my tomato plants, but, you were right.)
I’m just a little uncertain how this light will work out for lettuce: My LED grow light tubes (the ones made to be actual grow lights) have all crapped out, or partially so, so, using this new light - basically a very high output shop light with hopefully enough output in the right wavelengths, is a bit of a “guess”. I suppose if the lettuce looks spindly and weak, I’ll know it needs either more blue (or red?) light.