This is a most common question as many new artists do not yet trust their ability to choose colors. They are countless books written on color theory. Pages filled with long paragraphs about the nature of light, complementary and primary colors with a few images to brighten the dense text. If you have difficulty sleeping, they represent a good remedy. They are dull and will not answer your question, what color should I use?
When you get up in the morning and get dressed, you do not consult a color wheel to decide which dress goes well with which shoes or which pants best compliment which shirt. Instead (with more or less success or ability) you choose colors intuitively. When it comes to art, that same intuition that served you so well holds true. It is all you have, this innate ability to see that black and white are a safe choice, that green and red may be a tad loud and that a pale violet may compliment another pastel tone such as a pink.
Choose color intuitively and trust your first intuition. The colors that you choose correspond to your emotions and you are likely to pick different colors on a different day. You may choose grey when you are sad and yellow on a happy day. Yes at first, the color you chose intuitively may seem a bit much, so change it slowly. Add a little white to make it lighter, a hint of yellow to make it warmer and a touch of blue to make the color seem cooler. Keep adding as if you were cooking, a little more salt, a pinch of pepper… They are no set colors that you can buy that are sky blue or leaf green. And who says that the
sky must be blue? Choose a color that approximate what you want or what you feel, then change it until it pleases your eyes. Experiment, take chances but always trust your first impulse, the one that told you to paint a sky red instead of blue.
Essentially all colors got well together. Have you ever been in nature and thought that the color of a tree did not match the color of the grass? No, but in nature, colors are subtle and the details infinite. Every change you make to a color affects the other colors. So keep making changes, making sure that your entire piece espouses the same light. It is a choreography, be patient and make every part of your painting harmonize with the other. Trust your first colors and endlessly refine.
Forget techniques, they will come with experience, it is what you have to say that will move me. Children drawings are usually not great technically but they are often magical. You can see how skilled Norman Rockwell was as an illustrator but it is Van Gogh strokes, his colors and passion that touches your heart. Choose colors according to how you feel and trust your intuition.