Killerbee's shouldn't brown out, but they may lose a bit of their bright yellow as they age. My girl's spine is a paler yellow than her sides... This pic shows it pretty well.
Here she is at 500gms with her dad (bee)
Thats one of the prettiest Bees Ive ever seen.
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0.1 Pastel
1.0 Pastel
0.1 Fire
1.0 Bumblebee