I've been having a lot of trouble picking colors that work

A few pointers:

Value is more important than color. If you greyscale your image and it looks too light, too dark, or too grey, color is probably not the main problem. Nothing will look right if your values are too muddy.

When using complimentary colors, be wary of saturation. You shouldn't have your compliment be as saturated as your main color or it'll become an eyesore. A desaturated blue will still look quite vibrant next a saturated orange.

There's a book called Color and Light by James Gurney, it'll give you a rundown of the sort of things you should be thinking about.

>>2934674
Just feel it.

Each color has a specific value assigned to it. It is your job to have a good idea of what these values are.
Example, you cannot have a strong red hue when the value is almost white or almost black. Red's hue is the strongest when it's about a 60 or 70% grey. A good yellow can be like a 40-50%
Like the other anon said, usually if a painting isn't working, eighty percent of the time it's due to your value structure.

For colors specifically, a good, generic rule of thumb to follow is:
Local Color of Object + Color of Light = Light Plane
Local Color of Object + Complement Color of Light = Shadowed Plane
The Color of Light is kind of the "key" you want to work in that will affect every object in your scene.
I'm not making that shit up, Steve Huston approves of this formula. It's not the greatest, but it allows for a united color palette that works.

Also, look up the program Color Constructor. It does a more accurate portrayal of colors in a situation. The older version is better if you don't have Photoshop CC