When assigning the bog blight funerary mask there are no dye slots, but if you had a helmet previously dyed it will apply whatever was in dye channel 1. I had a helmet here with balckreach blue in channel 1, then previewed the bog blight funerary mask and it was blue. If you undo the funerary mask and clear the dye channel, then reapply the funerary mask it will be the correct default color, but when you save the outfit it will take whatever was in dye channel 1 before. If you clear the dye channel 1 with a dyeable helm, save changes, then switch to the funerary mask it will retain the default color after saving.

Without clearing and saving previous dye channels


After clearing and saving previous dye channels



It's also mentioned here in another thread.
https://forums.elderscrollsonline.com/en/discussion/comment/7238593#Comment_7238593