They probably won't turn brown but I'm welling to bet they'll get lighter if anything. I'm only speculating because I've never tried crossing them myself but the blue rillis I've seen/kept are not as blue as my velvets.
I have blue rilis and velvets together in one of my tanks. I haven't seen any darkening of blues so far, then again I haven't isolated a rili and a BV yet.
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