The 500 set improved multiple aspects from the older 302. Firstly, it used a self compensating network which adjusted for the distance between the telephone and the central office. On older telephones, longer distance would increase resistance and affect the loudness and transmission of conversations. The 500 removed this variable. It also moved the numbers outside of the dial, making them easier to read and more resistant to wear. It also introduced a more modern rounded shape.
Those colors included: black, ivory, dark blue, aqua blue, dark beige, light beige, dark gray, light gray, brown, red, yellow, green, white, pink, and turquoise. Custom colors, like lime and orange were also produced for special customers.
Besides Western Electric, other companies, like Kellogg ITT and Stromberg Carlson were licensed to produce similar sets. Produced over the span of 3 decades, the 500 type is probably the most common rotary dial telephone in the United States.