I am often frustrated with color theory instruction because it's usually relevant to painting or website design, and not so much for working with fiber. Unless we are dyers, we generally work with the colors as we find them. Not only that, but the texture of the yarn or stitch can affect the color due to the play of shadows and light.
I've adapted some of the color exercises I've found in classic color theory books for myself in my crochet designs. Feel free to experiment along with me in your own work.
The first exercise is using a monochromatic color palette; (using only one color). Here's a picture from Interaction of Color by Josef Albers:
For the first experiment, look over your stash (or some yarn in a shop), and pick out all the black yarns, or all the white yarns. Look at them all together in good light (sunlight is best), and notice the slight variations in color. Notice how the light affects the color of the smooth or glossy yarns in a different way than it does with the fuzzy or nubby yarns. Take out some different shades of gray, and arrange them in a pattern like the picture above. (If you are trying this in a yarn shop, don't forget to put everything back in the correct bins when you are finished!)
Try designing a project using only one color.
I once made a "dream cloud blanket," a free-form crocheted afghan, all in different white yarns. I made sure I used a lot of different textures of yarns--mohair to metallics--and textured stitches to keep it interesting. Another time I was inspired by a little girl who told me her favorite color was not pink or purple but yellow. I made a stuffed animal for her, a cat wearing striped pants and polka-dotted sweater, using every different yellow I could find.
Sometimes limiting yourself can push you in creative ways you hadn't expected!