You can "color by number" with your single head FDM 3D printer to make colorful projects that will leave people wondering how much you've spent on a full color 3D printer.
Step 1: Get a Filament Sample Pack
Unless you're a fab shop that has spools and spools of filament laying around. You don't want to be limited to the colors you have on hand. An easy compromise is to buy a sample pack of many colors. The only downside is that some sample packs can give you very small amounts of each color. Before getting carried away make sure you have enough of each color to do what you want to do.
Step 2: Google "Color by Number"
There are plenty of these available online, I'm choosing one that will showcase the near-limitless amount of colors that can be incorporated - a rainbow. If you followed the previous step you don't have to choose around what colors you have on hand.
Step 3: Transfer Your Design Into CAD
Some CAD programs make this easier than others.
This was a simple design so rather than tracing, I just drew a rainbow and some clouds from scratch.
Step 4: Extrude by Number
Rather than coloring by number, extrude by number.
In other words if the design says Purple = 1, Blue = 2, Green = 3... etc
Multiply Purple's number (1) by 0.02" and extrude the purple areas that amount (0.020")
Multiply Blue's number (2) by 0.02" and extrude the blue areas that amount (0.04")
Multiply Green's number (3) by 0.02" and extrude the green areas that amount (0.06")
Step 5: Time to Print
Start printing the lowest numbered color first, once you see that a color tier has finished, pause your printer, change the filament to the next color and then restart the printer. Repeat. This project took 7 tiers of color.
It might be a good idea to use a color you have alot of and make a single color test print so you can see when each color tier is finishing.