A hex color is written as a hash character, "#", followed by 3 or 4 hexadecimal numbers, say 6, resp. 8, digits (0-9A-F). The first 3 pairs of digits specify the red, green and blue components. When there are 8 digits, then the last pair is interpreted as alpha channel defining transparency, where `00`

represents a fully transparent color and `FF`

represent a fully opaque color.

The result will be returned as a matrix having 3 or 4 rows, depending on if the input contained a `RRGGBBAA`

definition or not. No distinction is made between upper and lower case. A missing leading # is tolerated.