The most famous racist in history (Hitler) used evolution to justify his racism (aryan supermen)when asked. Richard Dawkins correctly states that an evolutionist has no justification for declaring what Hitler did wrong that can be derived from evolutionary implications. With a title like this how could it not be used:
"On the origin of species by means of natural selection or the preservation of favored races in the struggle for life."*
This claim requires support.
(I'm politely saying it's bogus.)
Back to the OP.....some things are certain:
Atheism & agnosticism have no dogma whatsoever, so they don't push people towards or from racism.
Religions do have dogma, & some of them do or once did push racism. I'll skip examples, since there
would be great offense taken (supported by lame arguments that their religion is color blind, or the
the ole No True Scotsman thingie).
So I'd say that the faithful have a greater tendency towards racism. But this matters naught to me,
since I judge people by individual behavior. Religion tells me little about them.