The fonts data structure can't store color information, but you can use draw image function to draw color image.

You can also create one font character for each colour plane and "or" together a number of them.

See this thread and the tools they were using.

https://community.spark.io/t/digole-uart-i2c-spi-display-library/2392?page=20

I actually make use of user fonts and command sets to draw "sprites" and other graphical objects on my OLED displays.

I place my fonts in MCU flash memory and after the fonts, I add command sets to do the proper setups, like setting the font, the foreground and background colors, drawing mode, etc. I also have command sets to draw "strings" of characters, which are graphics from my custom fonts.

You can change colors between outputting characters, and there are commands to move the starting postion relative to where it currently is, so you can draw over previous outputs. I do at times arrange my font images to 3 bitmaps plus one mask. I mask out the area first, then draw 3 times in the same spot, the 3 bitmaps, changing the foreground color from red, to green, to blue.