Non-tattered sail texture: attached.
Different colours: get TX Convertor if you don't already have it. Convert that file to plain .tga format so you can read it into Photoshop, GIMP or whichever other picture-editing program you use. Change the colour to suit yourself, then use TX Convertor to change the new file to .tga.tx format which the game can read.
I don't know how to persuade the game to include these textures as options for sails, though.
I did a teeny tiny tweak a few days ago and added the sails (which look sweet, thanks), and figured it can't hurt to share. It doesn't look like the file was updated in a while so it should be safe to just overwrite. It goes into PROGRAM\INTERFACE