We used to assume that if the selected text started with "0x" when
inserting a color, we had to replace exactly 6 bytes after the "0x"
prefix.  Although this is generally the case as most color formats use
6 hexadecimal digits, it still would erase either too many or too few
characters if actually replacing something shorter (i.e. "0xfff") or
longer (i.e. "0xffffffffffff").

It could even partially override multi-byte characters if the 8th byte
after the selection start was in the middle of a character, as the
length was in bytes and not characters.

Fix this by honoring the actual selection end.