[Set]:`None` colour not accepted in `set colour`

### What Red version are you using?

3.5.22

### Cog name

Core

### Command name

set colour

### What did you expect to happen?

I expected that "None" colour for embeds would be allowed, tho it appears it’s not allowed by converter.

### What actually happened?

It said it isn’t allowed, tho we can use it when making embeds on our own.

### How can we reproduce this error?

1. Run `[p]set colour None` on your redbot instance.

### Anything else?

_No response_