Since we are on the subject, I found Cover Design Studio to be an interesting read in regards to their page on best colors for book covers. Below is a helpful reminder of the tone certain colors invoke when used in a design. If I were to follow this chart for my cover design then I would say dark red and grey would more closely align with the tone of the story compared to my previous colors of green and orange. 
| Red | Energy, enthusiasm, emotion, power | 
| Dark Red | Passion, depth, dominance, prestige | 
| Orange | Positive, dynamic, optimistic, confident | 
| Pale Yellow | Friendly, approachable, warm | 
| Bold Yellow | Ambition, motivation, creativity, cutting edge | 
| Green | Nature, vitality, environment, health | 
| Blue | Dependability, trust, thoughtfulness, calm | 
| Dark Blue | Deep sincerity, intuition, truth | 
| Light Purple | prosperity, spirituality, creativity, harmony | 
| Dark Purple | Depth, wealth, mystery, fantasy | 
| Grey | Sophistication, knowledge, prestige, wisdom | 
| Pink | Youth, playfulness, emotion, innocence | 
| White | Clean, straightforward, self-sufficient, simple | 
| Black | Authority, power, control, mystery, suspense | 
| Brown | Natural, of the earth, comfortable, organic |