I think a gold would look richer, contrast nicely with the yellow curtains. You could easily change the look to go with browns and aquas, if you start to crave neutrals.

I don't know that i would do yellow and i think you're right about the blue and red. If the couch is in ok shape i would maybe consider buying some pillows to add the color. Do you have a picture you could post?

I definitely agree-- you would be better off to add pillows and maybe a nice throw to pull in more of the colors and give it a unified look. It would be much easier with pictures but If you could find some pillows that have some of the accent colors in a pattern that would probably give you a much nicer look. I have a hard time picturing a yellow sofa let alone with yellow curtains as well. But remember you are there if you think it would look ok--who are we to say it won't. Stranger things have looked awesome irl.

ok thanks for the pillows idea! :)

Would a slipcover that isn't a solid color be acceptable? I don't exactly know what your style is but something like a ticking stripe in blue/white or even red/white with some yellow pillows would look nice. The white stripes would "mellow down" the color some. Overstock has a blue/beige/white stripe that might also look good; the blue in this one might be closer to the rug color. Surefit.com has a red/white/blue/green stripe on clearance (Avalon Stripe - the picture is better at JCPenney although their price is a joke) that I don't see evoking quite the same "nursery school" vibe that a solid red slipcover would.

Another option would be to go to a darker neutral that doesn't fade into the wall color - maybe more of a "toast" color - and pillows that pick up the other accent colors.

It's very hard to make WT recommendations without seeing the room. But instinctually, I have to say, don't "match" your curtains to the sofa.