That's an excellent question that gets to the heart of practical urban design. The short answer is a dark, muted green—often called "park green" or "forest green"—or a deep charcoal gray. These colors strike the best balance.
Here’s the reasoning: Lighter colors like white or beige show every drip, smudge, and stain almost immediately, making bins look dirty and unkempt. Pure black, while great at hiding dirt, can absorb too much heat in the sun and sometimes blend into shadows, making it less noticeable.
A dark green or charcoal gray is effective because it's dark enough to camouflage common urban grime like dust, liquid rings, and minor scuffs. At the same time, it provides enough contrast against common urban backgrounds (concrete, asphalt, brick) to remain visually distinct. This color family is also generally perceived as neutral, inoffensive, and somewhat "natural," fitting into varied cityscapes without shouting for attention.
For maximum noticeability, the color of the bin itself is less important than high-contrast, brightly colored signage and graphics on it. Think a dark green bin with bold, international "waste" symbols or bright lettering in white, yellow, or blue. This combination ensures the bin is easy to spot when needed, while the base color maintains a cleaner appearance for longer between cleanings.