Just to add a bit to this: Testing this also on Kubuntu 16.04.3 LTS in X
default color depth 24 bit and running apitrace on obs showed nothing
suspicious to me wrt. selection of fbconfigs and visuals, creation of
framebuffers etc. Just regular 8 bpc fbconfigs. The color corruption did happen
with the x-composite input plugin, but not with the xshm input plugin.

So the corruption seems to stem from the x-composite based grabbing/input side,
not from the output/rendering/display side.