Once I have a rough draft of the design I will take a digital photo and upload it onto my computer. I like to use photoshop to help choose colors that are pleasing. I had the idea to use only one color for the background but stumbled upon the idea of using gradations of the same color and decided that was much more interesting. I will play with colors using the "hue saturation" feature to see what other color schemes are pleasing.
The actual glass will obviously not be exactly as shown but you can get an idea of how the colors blend together. Here were two options that I liked but I chose the pink and aqua.