Completing a quest that is red (or orange) does give more XP. Much like killing a mob that is red (or orange) and how green mobs / quests give less.

I'll try to find a link that goes over this, but I remember in the original guides it would list "Max XP / Average XP / Lowest XP" for all the quests. Granted, that was from when WoW first came out though.

Some links

Mob difficult Colors
http://www.wowwiki.com/DifficultyColors

Quest XP
http://www.wowwiki.com/Formulas:Quest_XP

There was also a little trick I used to power level a toon of mine.

If a mob is green to someone, and red (even skull) to you, if you tag it solo, and they kill it, you get full rewards (XP & Loot). The XP does not degrade as normal gain rules.

I did this with my hunter some time ago and got, on average, 1 level an hour for my earily 50s by farming the red dragonkin in BS (I think - the ones for the Alliance Ony chain begining).