This tutorial will show you how to add very simple effects to your painting if you feel that the color is lacking or too plain.
The image above is the original version of a painting I made. Looking at it, I thought it was overwhelmingly brown and didn't stimulate the eyes enough. However, it would be a pain to change by physically repainting with different colors.
To start, I created a new layer above the painting and selected 'Linear Burn' in the effects drop-down to the upper left of the layers (left). I set it at 31, but you can adjust it as you like. It's not super visible when viewing in this manner, but the linear burn saturated and darkened the colors throughout the painting. If increase the level, it can bring out undertones of other colors--in this painting, it affects it by bringing out some purples and blues in the clothes and reds in the background.
You can leave the effects here, but I wanted to create more color.
Here, I made another layer and set it with 'Color Dodge'. As you can tell, it seems to bring out bright colors in the painting. I assume that these are colors that are more hinted at in the original--the color dodge just makes them pop and glow. This new layer is placed below the linear burn layer; it does change the painting if it is placed above, but whichever placement is fine, depending on the result and what you are looking for.
The last detail was much more inconspicuous, especially when the painting is viewed as a whole (as smaller details often are, to my great disappointment). I created a layer above the painting but below the two effect layers. Knowing the effect of using the 'Lighten' setting (brightening), I chose a very bright, very saturated yellow color and painted gold drips, or tears, on the face. When paired with lighten, the color would fade and glow as you see it doing above.