what would cause the tips of green beans to turn brown when steaming.

Maybe the tipes should of been trimmed to prevent over-steaming/cooking.

Rather than steaming, try a quick blanche in salted water (30 seconds or a bit more), then plunge into an ice bath to stop the cooking. They will now be a vibrant green color. When ready for service, sautee in some butter....yum.

Did you squeeze lemon juice on them? Acid can cause browning. Overcooking can also cause browning.