Hi,
It's the old Read-Modify-Write problem. HIGH/LOW takes a little longer to execute (because it also writes to TRIS every time) so it doesn't suffer in this case but direct port.bit access may suffer from the RMW issue.

Try writing to the port latch instead of the port itself by aliasing your YLED/GLED to LATA.0 and LATA.1

/Henrik.