I came across Creating Gedit Schemes:
Gedit gives specific colors to the variables and another color for the commands. Many parts of the programming syntax has a special color. This programmer can immediately identify parts of the code by seeing the color. However, what if the programmer is color-blind or does not like the way some of the colors look? Thankfully, a simple XML file can instruct Gedit how to display programming syntaxes and plain text.