7 Wonders: Why do resources on the board have colored background?
From the rulebook:

The resources a player can buy from their neighboring cities are:
• the resources initially produced by the city (as indicated on the board)
• the resources from its brown cards (raw materials)
• the resources from its gray cards (manufactured goods)
However, it is impossible to buy the resources produced by some commercial
structures (yellow cards) or by some Wonders: these resources are
reserved to their owner.

Since resources initially produced by cities behave identically to resources produced by brown/gray cards, it is good enough reason to color them same way, to distinquish them from non-buyable resources.