I do not profess to be an expert in color theory. I usually just pick the colors for my quilts based on the mood I am in, the colors I like, and whatever I find pleasing to the eye.
But I have found that knowing a little bit of color theory can help when picking out fabrics and colors. You don't want your fabrics to all mush together. Maybe you want a specific part of the quilt to pop while you want another part to kind of blend together. Knowing basic color theory can help take out some of the guess work as you put your fabric selections together.
Here are some tips and definitions that I found helpful.
Primary Colors - These are the colors you cannot create by mixing other colors together. They are red, yellow, and blue.
Secondary Colors - These are colors you create by mixing together the primary colors. They are orange, purple, and green.
Tertiary Colors - These are colors you create by mixing a primary and a secondary color. They are usually named after the two main colors that are mixed together, like blue-green for example.
Color Wheel - A circular way of showing the relationship between Primary, Secondary, and Tertiary colors.
Value - Value is basically how much black or white is added to the color. Tint, Shade, and Tone are usually words that describe the value of the color. Tint is when white is added. Shade is when black is added. Tone is when gray is added.
An easy way to see if the fabrics you have chosen are the same value is to looks them through your phone's camera on a gray scale setting.
Warm or Cool Colors - Warm colors are usually colors that have a lot of red, orange, and yellow in them. Cool colors are usually colors that have a lot of blue, green, or purple. These colors usually elicit a feeling of warmth or coolness. You don't have to stick to just warm or just cool colors in your quilt.
Color Relationships - This basically describes how colors "play together." If you put a blue and a green together they will play well, but maybe the blue will blend in with the green. But a red added to it would really stand out with the blue.
After you pick your fabrics, take a picture of them from far away. Or look at them while squinting. This will help show which colors stand out and which ones blend together too much.
If you want to experiment with colors and color combinations, I find scribbling with color pencils, or playing with other small items to be satisfying. I also have a Pinterest board devoted to fun color combinations that I find.
Some good tools for color experimentation is Katie Fowler's Foolproof Color Workbook, and Foolproof Color Wheel Set.
Have fun quilting!