The reason why you need an underbase when printing your design on dark colored tees is because screen printing inks are translucent. Some inks are more opaque than others. Check out the image above and you will see that the colors are not vibrant. This is because the purple color of the tee shirt is peeking through the printed colors. Whites, blacks, and reds tend to print more opaquely and that is the reason why they do not look so bad. But the yellows and light blues in the design have changed altogether; due to the tee color mixing with the ink. This often creates terrible results. Especially if you have complimentary colors, such as green printing on a red t-shirt. Or, even worse, light colors printing on a black garment.