Still fairly new and I think I messed this up while playing and wanted to correct my mistake if so. Card ability went off at the table tapping all non-white creatures. Other players weren't playing any white- so easy card effect, but I thought that since I was Red/White the effect would sort of cancel out. After reading about multi-coloured cards being fully each colour represented on the card, that's where my doubts come in.
Official answer is?
Non-white means "not white", and creatures that are both white and red are white, so they are not nonwhite. Your effect (Ivory Giant or something similar, I guess), would thus only have tapped your creatures that are only red or colorless. Those that are both white and red don't get tapped.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Official answer is?