Dual Color XYZ 20mm Hollow Calibration Cube (0% infill possible)

This is a remix of my own design of the XYZ 20mm Hollow Calibration Cube, with following changes:
- dual color: body & letters, as separate STL files or combined in AMF
- X' and Y' at the opposite sides of X and Y letters

The prints in the photos are done
1) Cyan/Yellow with Diamond Hotend sliced with Slic3r
2) Violet/White with Dual CR10 Hotends sliced with Slic3r-PE
3) Various color Z transitions with Diamond Hotend sliced with Slic3r-PE