I hate to waste this paint that is already bought and paid for. Can I salvage it by adding another color? I really want to reduce the 'highlighter' yellow tone it has and make it more of a buttery yellow.

Any advice?

I've also found that yellows look less jarring after two or even three coats of good-quality paint (counter-intuitive, I know - don't ask me to explain). Since you've already started and have the paint, you might want to see if you like the color better after two or three coats.

Also, give the color some time for you to adjust to it, and remember that the room will look quite different when your furnishings and wall coverings are back in place (less exposed wall area, different light). I painted my home office BM Hawthorne Yellow and the room seemed to explode, but once I put my cherry furniture and cherry-stained interior shutters back, it was much more subdued. (The room has four windows on two walls, with a south/west exposure, so the wooden shutters made a huge difference.)

Finally, if you still hate it and don't want to re-prime and re-paint, you might be able to use a glaze or faux finish technique to tone it down. Post a photo of your room and the name of the color you used, and someone here (not me!) will probably be able to suggest a color and technique that would work. Be sure to test it on a LARGE piece of drywall or posterboard before trying it on your walls.

P.S. If you decide to change color, you can give away your unused paint on Freecycle or Craig's List. Some non-profits may also be willing to take it (but usually will take only full gallons).