Well, I went a little crazy with Ceterumnet's iPhone case, thing #22572. The images are roughly in the order they were created. This is absolutly inspired by RichRap, who was the first that I saw do live filament swaps. Most of the credit should go to Alexrj at slic3r. org for All the cool patterns in Slic3r.
I modded the Greg's Wade's Adrian's extruder so that I can switch filament on the fly. I basically made the hole below the hobbed bolt into a funnel, so that the Cut-N-Follow filament switch would work. I did this with a single extruder, 3mm filament and a 0.46mm hottip. I also uploaded my modded Greg's with a Budaschnozzle mount because Thingiverse requires a model to publish.
I only print in PLA because ABS is bad for the planet. It's also pretty.
If you use this technique, please upload your images as a derivative. I can't wait to see where people go with this.
I use Slic3r to slice my files, which has five built-in solid fill patterns. I'd always used concentric, so I started by stumbling onto two color concentrics. After that I tryed all the other fill patterns while switching to smaller and smaller pieces of filament, down to about 1" reliably now.
Switching to transparent colors lets you see the layer below, so I started swapping in clear pieces every other swap. This gives the cool plaid look with the rectangular infill option at 100%. Transparent PLA colors can also mix colors, so you get some beatiful effects. You should also play with the infil angle for fun.
Next I played with the Hilbert Cruve solid surfaces, which is quite striking in person. Oohs and aahs all around. I let the parimeter be normal Z height (Prusa!), then after the outlines are done I manually lift the Z about .5MM so that the filament is coming out round on the heated bed (required!). Then I manually lower the Z on the second and consecutive layers back to normal smashy smashy. This lets the different color second layer show and accent the Hilbert curve.
Another interesting style is to use the Hilbert Curve (or other) solid surfaces, but make that whole layer clear and bumpy (raised Z first layer) for 3d touch effect.
Then someone suggesting cutting logos into the back of the case. I made the case 1mm thicker, then did boolean cuts with extruded DXFs I downloaded form the web (non-commercial!) By cutting the logo 1mm deep with 0.30mm layers, you can get the outline, surface fill, one infill layer, then the backing bridging for the logo, so you can get lots of color changes.