ULN2803 not working like I expect, or at all
Ah, ULN2803 is a 8-channel (low side switching) sink driver, for common anode LEDs.
For common cathode LEDs, you need the complement/mirror: UDN2981 (high side switching) 8-channel source drivers.
The following schematic might help to tell the difference between high side and low side switching.



Appendices
Appendix A - 7-segment LED Circuit Design v0.1



Appendix B - Using software switchable PSUs to replace UDN2981
Idea: one switchable PSU per LED.