If you're using Dungeonbuddy, there is a setting under the Bot Config menu called Maximum Item Vendor Quality.
I believe;
- Poor = Grey
- Common = White
- Uncommon = Green
- Rare = Blue
- Epic = Purple
- Legendary = Orange
- Artifact = Don't think these even exist in-game
- Heirloom = Gold
So if you want to keep all greens and blues, set it to Common.