Here’s the gist: You can never park at a red curb, and white curbs are (almost*) always for passenger loading only. But green- and yellow-painted curbs are those inscrutable spots that people often feel too afraid to use.
* Pasadena (6am–6pm) and Burbank (8am–6pm) both have set enforcement hours for white curbs. However, there are so many caveats that it’s highly unlikely you’ll find a white curb where you can legally park.