I took everyone's advice to play around with color so added in some of the same material that I'm going to use for grass around the base of the house. It looked better, but the whole thing still wasn't working for me. It's just too big, too much, too prominent, too...something.
The color still isn't right, and the plants need to be smaller, definitely. Small sedums in scale would probably be flocking, and I have some flocking, but I don't like flocking, it looks too fake.
I ripped the plants off the roof, then played around with shorter iris, moss and other various smaller materials, but nothing worked. I'll try planting a thatched roof later, on a different house, after I've given it some more thought and experimentation...or maybe on a half scale house flocking would look fine. I don't want the roofline to be the focal point of the whole project, I want it to blend in. With the Spring Fling deadline looming over me I don't have time to figure out a planted roof right now.
Perhaps it's a matter of perspective...when I walked around the real house I had to crane my neck to stare up at the roof, while I'm looking down at the roof on the miniature. Maybe a planted roof just isn't going to translate to miniature.