Regarding Black Douglas vs Red Douglas and Douglas tartans, I would ignore the Black Douglas tartan as it is not one recognized by Clan Douglas, to the best of my knowledge, and except for relatively lightweight Dalgleish tartan is a special weave (read costly and long wait just for the tartan). The Douglas fortunes changed between two brothers, one known as the Black and the other the Red, a couple times because of differences in support of different monarchs during times of dispute over who was king, with the Black eventually winning out. But that Douglas Modern tartan with the white stripe (not the red stripe) is the standard for the Douglas clan now, although there are several interesting variants (I collect Douglas tartan kilts--red, black, brown, ancient, modern, weathered, etc...).
Mini Hijack to my own thread here- FM, this is very enlightening. I'll pass this on to my mum. I'm fortunate you answered this being a Douglas Tartan collector (Forrest is sept of Douglas, correct?)