You're the one who signed up for beta. (And should have known what you could be getting into)
If something as insignificant as folder icons really gets your blood boiling, then perhaps beta isn't for you. Especially a somewhat early build of the beta. It's expected to have hickups.
Send feedback with that image and restore back to 10.10.4 (or just live with for potentially one more week when the next build could come out and be already fixed.)