Hi sterretje, thanks for your reply.
Yeah, I noticed that the hex value changes when I type chars in the A/a-F/f range... but I thought it was just my imagination.
I supposed that strings (array of chars) are interpreted byte-by-byte in any case. I was wrong.
Any workaround? What I could type in the PROGMEM section when a|b|c|d|e|f char is following my hex-code?