This is a question best answered by another question … What colors do YOU like together? When choosing what fabrics and colors to use for making doll clothes (or when sewing any project at all) I usually spend a bit of time 'playing' with color and print combinations to see what I like best, or what achieves the look I want ie. subtle or dramatic, pretty or startling, fun or more traditional.
If you really do not know where to start when combining colors, you could refer to a color wheel. This is a way of visually seeing what colors 'work' together to create harmonious color combinations. Technically speaking, complementary colors are any two colors opposite each other on the color wheel, any three colors equally spaced around the color wheel forming a triangle, or any four colors forming a rectangle. Colors beside each other on the color wheel also work well together. Then there are all sorts of variations within these combinations.
So rather than worry about what colors 'technically' work well together, simply 'play around' with the colors you have available to see what YOU like, or if you are making doll clothes for your daughter or granddaughter, why not ask what THEY like . Doing this can result in some really groovy combinations as kids are very open to any color or print 'working' together. I particularly like pinks and purples and have made many beautiful doll clothes out of these combinations over the years.
Another way of using color combinations is to pick just one color from a print and highlight that color in the trim or as the matching piece of clothing eg colorful print top with pants in a single color from the print. In reality the fabric you can find or purchase may also limit your selection of colors for an outfit. You might even want to go with a specific Theme, for example Dinosaurs. Even then colors can be very important for the final exit when making your doll clothes.
At the end of the day the best advice I can give regards what color combinations to use when making doll clothes, is to know what colors YOU like and do not like, then you can not go wrong with the colors that you choose. The most important thing is to have lots of fun mixing things around and enjoy trying different colors and creating beautiful outfits.