As always, you're the best! Got it!One cable regardless of color. Marking the ends with colored heat-shrink if you make the cables or using quality electrical tape is perfectly acceptable. If you have a bunch of black cable as others stated: marking the terminal ends is fine.
I would be ok with all black cables but I’d never use red for a neg(-) connection, and probably 100% would make up your circled series cables from black.
Electrically it doesn’t matter but for safety and ease of service purposes that’s what I’d do.