You're funny there mate 
Blood gore and dark colours, don't set a dark tone for you, sure, no problem, I can accept that. DA:Origins being merry? you're talking out of your ass. Starting with Duncan, and the King, Alistairs brother dieing bbllody deaths and betrayed, ending for some with losing morrigan no matter what if she was the romance. My Warden DIED at the end Origins, and for him not to die, either the other good guy dies, or one of you does the ritual to have a baby/maybe even your own baby, become "posessed" by one of the darkest beings/powers in the world of Thedas. Or maybe take the bad guy and let him be used as "live bait" and bite the dust... not very happy high fantasy.
Lets not forget the fact that every chapter is also about racism, violence, rape, slavery etc. Kill your dwarven firend's love, or kill the righteous golem trying to do what is moraly right. Go against one of your best friends will and force him king? or force him to marry a girl he dosnt want to for politics? Kill a possesed boy or try to save him and do a dark bargain, or kill the deamon, while still facing the consequences?
There are plenty of other examples of how dark, and not a happely ever after a story Origins was.
I din't say DA:O was "merry". I said it was optimisitic. And in the end, it is. Yes, bad stuff happens. Yes, the setting and the opening of the story are dark. But from that starting point of a crapsack world in a bad state, the plot is a an optimistic, idealistic story of heroism triumphing over darkness. From that starting point things went about as well as they possibly could have. Is everything sunshine and rainbows? of course not. Are there some cases where there is no perfect option? Sure. And, yeah, you have the option for doing less heroic things if you chose, but there are no consequences for not doing those things. If you do the heroic thing, the "good" thing, it works. Ultimately, if you play that way, the warden and their merry band of companions defeat the Blight and save Ferelden (and Thedas as a whole) at a remarkably small cost (particularly when you look at previous Blights. The Fourth one, as desribed in Last Flight was orders of magnitude worse than the Fifth, and that was still much much shorter than the earlier ones.) And that, frankly, is not "dark" or "realistic" (again, that's not a criticism, optimistic stories can be good, and Origins' is).
A dark story would have you trying and failing to save people time and time again (whereas, in DA:O, if you try to save someone, the vast majoirty of the time, you can by taking the right option). A dark story would leave many of your closest companions dead. A dark story would not give you a get out clause to avoid the sacrifice. A dark story would not give you "everthing turns out OK" opions like making peace between the Elves and Werewolves, or saving Connor by getting the mage's help. Hell, on the subject of mages, a dark story would have siding with the mages in the tower result in releasing a daemonic infestation into the world. A dark story would not have the Blight ending in a single game. You break the siege of Denerim perhaps, drive the darkspawn off temporarily, but the blight will last decades if not centuries. A dark, realistic story would not give you nearly as many choices in your actions. It would force more things upon you as one man cannot shape the world, they can only shape themselves in reaction to the world. The entire idea of a Hero saving Ferelden is hugely optimistic and idealistic.
So yeah, I stand by my assertion that Origins is a positive, optimistic story in a dark crapsack world.