Maybe something to do with picking mates (the redder the better), to warn off enemies (I don't taste good), or something to do with camouflage? Pretty general reasons for why animals are certain colors, probably none of which are the correct answer to your question.
True red tails(bcc) have a behaviour called caudal luring.... They twitch their tail out in front of their strike zone to lure in potential prey....
So you might ask yourself why red??? It has been proven that both bird and primates associate red with the colour of food..... if you imagine being up in a jungle canopy(bcc are very arboreal)... with all the green and brown... these few red blotches sure will stand out.....
Several of my boas actually still do this behaviour....
This would explain why boas from savannah regions dont have bright red coloured tails.....