I have DIY led lighting with multiple channels at different wavelengths. I can turn the whites and reds off leaving only the royal blues on (450ish nm) and make my plants pearl if i turn them up to 100%. Both plants and corals have chloraphyll. Sure there might be an optimal balance in spectrum and I doubt my plants would be happy with only 450nm for a long period of time(the fish were freaked out too) but it's not like plants won't grow from a reef light.
I think a 20k light over a planted tank would look terrible though. Especially red plants. A 10k light would be ok if there isnt too much PAR.