27th Street (detail) #2

Sooo… this is random, but:

I get the granite curbs (durable, better than concrete, whatever…) and the stone streets (historically, what else would you make them out of?) and the brick sidewalks (easy to repair, re-level, more affordable than stone, etc.)

But what’s up with the odd yellow/tan bricks at all the alley entrances/curb cuts? Is it to make them more visible? Are they a different material? What’s the reason for them?

(Sorry if this is a really trivial question, but I’ve always wondered)