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.