Why would color blind people struggle with this sign? There are no similar looking signs which mean something different.

The closest one would be this one:

And any color blind person is able to distinguish those two easily.

I see how it can be confusing for someone not used to it but for anyone who grew up in a country where this is the default it is perfectly understandable.

Nope:

https://en.m.wikipedia.org/wiki/Prohibitory_traffic_sign

Many countries use red circle + symbol to depict who is not permitted to drive there.

Why would you need curbs to be marked?

Over here in Germany parking 5 meters in front of crosswalks is illegal. No curb is ever striped, anywhere. If you are unsure whether you are too close you are expected to measure.