HueForge
/HueForge is amazing. It lets you care 3D printable pictures from images.
The original image is on the right. The predicted print is on the left. On the bottom left you can see the four colours I’m using (black, blue, grey and white) and you adjust the sliders to determine the image colours they match. It works wonderfully well and drops out an STL file. You just have to tell your printer the layer numbers where the colour changes and away you go.
If you have a 3D printer that can print different colours you should get HueForge. It’s as simple as that.