I've been able to reproduce this time and again....
creating iso tiles for a tilesheet I keep getting the error where I set a color with RGB 250,0,250 with opacity at 160; save it, reopen the file and it is now RGB 251,95,251 with opacity at 255 with PNG files.
this never used to be a problem (of course this may be a windows 8 thing but I don't recall having this issue with earlier versions of PDN on Win7.