Value is the most important part of a painting. If you get the value right everything else will fall into place. There are certainly a lot of things to consider including composition, balance and subject matter. But getting the colors right is also something important to consider.
You can't take color and use it straight from the tube (most of the time). Usually there is some blending involved. For example, you can't squeeze some green out of a tube for a leaf color. There are many colors in that leaf besides green. Also, you can mix up your own green, so a tube green isn't necessary. But I digress.
Going back to the leaf color, you would first mix blue and yellow to get green, is there light, dark, shadow? That would change the value. Is there a flower near the leaf, that would change the color. How about the bark of the tree or the sky? All of these things affect the color of the leaf. There are lots of things to consider.
Another thing to consider are paint recipes. For example, what is the color of a wave at it's crest where the light is shining through? How do you make a bright orange or mix your own cobalt blue?
Below I will share a short list some of my recipes that I either came up with myself, looked up on the internet and experimented with, or found in a book. It's up to you to decide the amounts of each color to blend together because it all depends on what you are painting.
Light in the ocean wave: ultramarine blue & turquoise blue & cadmium yellow
Cerulean blue: white & phthalo blue & black
Cobalt blue: ultramarine blue & turquoise blue
Bright orange: Cadmium yellow & permanent rose
Green apples: Cadmium yellow & black
Bright green apples: phthalo green & cadmium yellow light
Red apples: Alizarin crimson & Grumbacher red (or any permanent red)
The Green part of the apple near the stem: white, cadmium yellow light, yellow ochre, small amount of phthalo green