A 3D printed two colour sign for steam engines, 'Boiler Empty'. The one shown in the photo was printed at 0.8 size.
For those, like me, who have a single head printer but have not yet printed in two colours, this is what I do.
Print in white until the first layer that prints the lettering. Press 'Pause Print' and then click on the 'z' axis button to raise the head by 10mm. The printer should finish the instructions it has already sent, then will stop printing and raise the head. Quickly put a piece of paper between the print and the head. Using the buttons in the printing program, retract the filament, then pull back the pinch wheel and remove the white filament. Replace with red, then use the buttons again to extrude filament until the new colour comes out. All this should be caught with the paper. Clean the nozzle with tweezers, then remove the paper and remove any slight 'blob' on the print where the print stopped. Make sure the nozzle is still clean, then 'Resume Print'.
The print head should go back to where it stopped, and should seamlessly print in the new colour.
Obviously this will depend on the printing software you use. I use Repetier with the Cura slicer.

Now added a new file with deeper letters. If you resize the original too small, there will be very few lettering layers. The latest file should get round this.