They paint them red specifically for that reason? I didn't know that.
Colours do change over time, for some reason. Blood Angels used to have gold/yellow chest eagles with yellow and black warning stripes on their weapons sometimes. Now the chest is black/grey.
Also the trim on their shoulders seem to change at almost random. Took me ages to seek out as many pictures as I could and finally arrive at red trim as the official colouring :P
Unfortunately GW overlook quite afew "minor" details like that. Like in the latest Chaos codex, they took old pictures of certain named renegade chapters and gave them new names. So now the Warp Ghosts are now the Deathmongers, and The Damned Company of Lord Caustos is the Night Stalkers.
Trivial, really, but annoying nonetheless.