Color harmonizes by mixing complementary, analogous, and monochromatic color schemes and then using shades, tints, and tones add impact. Being aware of these relationships aid the designer in creating t-shirts that sell versus ones that sit on a shelf.
Some people like plain t-shirt graphics, some want super colorful graphics, some want a vintage look. Either way, knowing who the target is, and predicting what they will be drawn to is of utmost importance. I've worked on tons of t-shirt designs in Adobe Illustrator and Adobe Photoshop, all the way to print production and distribution.