Cheap is fine.
Royal blues will work, but I think throwing some red(630nm to 670nm) in there would be good, along with some white light (6500k). If you are planning on purchasing the sea lettuce from us, I think I do have some old 3W LED's in these spectrums I can throw into the box (For legal reasons, we will not warranty, guarantee, or endorse using these starpad 3W LEDs). Be sure to leave a comment if you want them.
If you look at the photosynthesis spectrum for Chlorophyll a and b, there is quite a bit in the red spectrum(630nm to 670nm). NOTE: This graph does not show the relative content of chlorophyll (b to a) in the macroalgae.
I think you will have some success with the AI sol blue, but I think you would have better luck with a full spectrum COB unit off of amazon (if you are okay with Non UL/ETL/CSA approved fixtures). If you want an approved fixture, one designed for saltwater environment and tuned for macroalgae, I would try to sell the AI sol blue and purchase the AI FUGE light (released at the end of the month).
Chlorophyll Image Credit:
By Chlorophyll_ab_spectra2.PNG: Daniele Pugliesiderivative work: M0tty - This file was derived from: Chlorophyll ab spectra2.PNG:, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=20509583