I generally favor pastels over the bright clear colors. The exception is red, which I really like, but Frilly you're right, it has to be a good clear red, not pinky or fuschia. That red Knockout rose really is a terrible color. If they were mine, I'd prune them back, dig them out and set them in pots beside the road, with a sign "Free Knockout rose bushes". Somebody will adopt them, I'm sure.
My real dislike are the plants with foliage that looks brown, or orange, or yellow or light green. Some of the fancy Heucheras that are available now are just awful looking. They just look unhealthy or dead to me. Purple foliage can be really gorgeous, but truth be told down here most of our purple foliage plants grow in the shade, so they look beautiful in the softer light.
The other one you really have to watch is the blazing bright yellow - it really clangs to me. Doesn't go with much else except maybe orange and if you happen to have it blooming with reds, your whole yard screams "McDonalds". Golden yellow is much more mellow, pastel yellow is one of my favorites but . . .
I'm looking out the window at a self-sown Tithonia plant that has just come into bloom:
- Ack.