I like to use white for 1, grey for 2 and red for 5, and then use other colors according to how the money is colored in whatever particular game/ set/ variant I’m working with. (e.g., when using Monopoly money, I go with yellow for 10, green for 20, blue for 50, etc. When using A&A IPC’s, I use red for 5, blue for 10… Come to think of it, when using Attack! money, I go with yellow for 1, tan for 2, red for 5, orange for 10… And I’d already just suggested using green for 1 with A&A IPC’s to match the money…)
Anyways, yes, I think I’m beginning to agree with you that putting a # on them just limits the flexibility of their uses.