Capture Date rule: is it time to retire it?

This is the difference:

In your example you could not enter the picture because of the inherent nature of your picture, ie, more than one color.

In a date limited entry, the pic would be perfect but cannot enter not because the picture did not meat the requirements but because it was taken before the host thought up the challenge.

The difference is essentially this.

Entering the picture of a horse in a challenge that asked for a pic of a cat is clearly wrong, vs, entering the picture of a horse in a challenge that asked for a picture of a horse but not being allowed to do so because you took the pic of the horse a day before the host announced the challenge.