Values First: Why Your Color Looks Off
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Color gets the blame, but value does the work. A painting with a strong value structure reads even in greyscale; a painting with beautiful colors and weak values falls apart the moment you desaturate it. Build value first and color becomes the easy part.
What value actually is
Value is how light or dark a color is, independent of its hue. A saturated yellow and a pale grey can share the same value even though they look nothing alike. Your eye reads value before it reads color, which is why a value structure is what makes an image legible from across the room.
Block in three values
Work in greyscale to start
Turn off color entirely, or paint on a greyscale layer. Removing hue removes the temptation to fix a value problem with a color.
Commit to three values
A light, a mid, and a dark. Assign every shape to one of the three. Squint at the canvas; if two shapes merge when you squint, they share a value and you have lost a read.
Protect your extremes
Keep your very lightest light and very darkest dark small and reserved for the focal point. If everything is high contrast, nothing is.
Squinting throws away detail and color and leaves you with value masses. If the image still reads when squinted, the value structure is sound. Do this constantly.
Add color on top of a solid value structure
Once the greyscale reads, bring color in on an Overlay or Color layer, or paint over the greyscale sampling hues at the value you already established. Because the values are locked, you are free to push saturation and temperature without breaking the image. This is why professionals so often start in monochrome: it separates the two hardest problems so you solve them one at a time.
Try more in Digital Painting; the related Layer Blend Modes for Shading, Explained; the layer modes cheat sheet.