Allow usage of two-digit color codes

When using a two-digit color code, the two digits should be used as the same value for R, G and B.

For example:
- `#13` has the same color result as `#131313`
- `#f9` has the same color result as `#f9f9f9`
- `#33` has the same color result as `#333` and `#333333`
- `#cc` has the same color result as `#ccc` and `#cccccc`
- etc.