Is it too hard to use another color?

The reason for trademarks is simple: branding. Since there are a near infinite number of colors available (for example: #e21088 should be different enough to not violate trademark), different groups can claim colors of their own (if they wish to trademark a certain color).

Catch-phrases, simple words ("Taste the Rainbow" or "They're more than Good, they're Great"), or even names (Mario) can be trademarked.

Furthermore, Trademarks only apply within the industry. I can use FedEx-brown so long as I don't compete against FedEx.