When GUI updates (close, change focused window with opened Mixer GUI) brightness value resets on previous value (before starting mixing). So when I choose color, start mixing, at the end I get wrong brightness value in can, even if I didn't close GUI.

Looks like problem shows your self on sending color to server.

MC version: 1.7.10
Forge version: 10.13.4.1492
OML version: 0.8
OB version: 1.4.4

As a workaround. After choosing color with gui, I'm changing hex value.