I put portable PNotes on USB disk of disk type: 'Removable Disk'.
When I clicked New Note, it show red background, or sometime dark purple background.
If I copy whole PNotes folder to disk type: 'Local Disk', it show yellow background.
Check whether Preferences-->Skins-->Use random background color is checked in portable version and in local one.
No. I use skinless and uncheck random skin.
Portable version and local (desktop) is exact same copy,
just in different disk type (shown in my computer).