We often take for granted that we view the world in such a wide variety of colors. There are so many different shades, hues, and tones, and only about 30 different names for colors we use in common vocabulary. If we were to ask you to imagine the color red, your version would be slightly (or even drastically) different from another person's imagining.
In fact we really can only understand colors as they relate to one another. Our minds use colors as reference points to each other to create a whole image. The result is that sometimes colors actually appear differently depending on what they are paired with. Observe the center square in the following images.
The middle squares above remain the same for each example. Does your brain seem to perceive the colors differently depending on what surrounds it? Does one middle square seem darker or lighter depending on what surrounds it?
Check out these different examples.
Do these middle rectangles appear to be the same color at first glance? Is it easier to see their difference when they are next to each other and surrounded by white?
The point is that colors are less constant than we would like to think. When choosing colors for a custom shirt it is important to keep in mind ALL colors in the design, specifically the ones that touch each other. In the Design Studio we have our standard list of colors, but if you have a specific color in mind the easiest way for us to understand exactly what you want is to use the Pantone Matching System (PMS) which provides a number for each color.
Also keep in mind that when designing custom shirts online it is a good idea to turn up the brightness on your computer monitor. This will help you to see more of the contrast between the colors you are working with. Have an awesome time creating a custom shirt!