You can get to this part of the system by clicking on the gear-shaped button on the right side of the Admin Toolbar and clicking on Settings in the droplist. Then click the Website Settings tab and the Calendar and Classes settings will load.
In the Calendar section you have the option of changing background and text colors in your Full Calendar View, as well as the text size for your Events. To change an item, simply click in the box for the option.
If changing a color, use the full spectrum color slider in the center to find the major color area you want to work with by clicking and holding between the two arrows and dragging your mouse up or down to the color you want. You will see the main color area change as your mouse slides. To pick the color you want, find the white circle in the lower left and click and hold on it with your mouse. Drag it around the color area until you see the color you want to the upper right. When you have the color you want, click the rainbow button in the far lower right of the color picker popup window.
As an alternative, if you already know the RGB or HSB numbers for your desired color, you can enter them and get your color that way. Or if you know the 6-character Hex code for your color you can enter it in the box at the bottom next to the number sign.
Once you have your Calendar configured the way you want, click the Save button in the lower right and go back to your Full Calendar View to make sure it looks good. You may need to go back and forth a few times to get it just right.
You can set to Sort Classes By a variety of methods. If you choose one that doesn't have Category in the title, the Categories will not display, so those options make the most sense if you only have one Category of classes. Please note that with the Custom setting, you can drag and drop the classes into whatever order you like, but Categories will not display.