Is it OK to paint bedroom plain yellow?
I just painted my two youngest boys' room an intense goldenrod. It is a small room, and with only one window and slanted ceilings, it tends to be dark and closed in. I wanted to brighten the room, but I can understand that if that wasn't the desired effect the color can be overwhelming. 

I personally find yellow to be a difficult color to tolerate for long periods of time; I believe that studies have shown that yellow can make people feel anxious. Here's a link http://www.colorpsychology.org/yellow/ that provides more insight.

If you already know that you don't love the color but can't afford to waste the paint, I would suggest you take it back to the paint store and ask them to tone it down for you, rather than just add white. Most paint stores or counters in big box stores will do this for free, however there is some skill involved and you might have better results at a store that exclusively specializes in paint.

To a certain extent, yes, adding furniture, paintings/photos, and curtains will break up the color and reduce the feeling of being inside an egg yolk, but, if it's enough to make you comfortable w/the color is questionable. The yellow may very well make some of the colors of other items look different; this may be good or bad. 

You might try changing the lighting in the room. If there are windows, try diffusing the light with sheer curtains or shades. Light bulbs, both LED and incandescent, are available in "warm" or "cool" tones that might make the room feel totally different. 

For some tips on how to decorate to "harmonize" the color, check out the Houzz app, which allows you to search through thousands of photos of homes. You can type in "yellow walls" and it'll show you what others have done, both professional interior decorators and regular folk, for inspiration and solutions.

If nothing helps, don't despair, I've made plenty of bad color choices in my day, and on a tight budget to boot, and I'm still sane (I think). I just wait a while and then repaint as soon as my budget and schedule allow it. That's the beauty of paint vs wallpaper-it's low commitment.