In most(all?) states, a medical signed diagnosis from a doctor as to what your disability is. Presnt this to the DMV and they should issue you a permit.

Heathrow Airport's Parking does not require an ability to see color. Color blindness is a mild disability and typically reds and blues are altered so I don't think purple would be an issue per say.

Yes, you can sue a bank for discrimination against disabled people under the Americans with Disabilities Act (ADA) and other related laws. It's important to gather evidence of the discrimination and consult with an attorney who specializes in disability discrimination cases to assess your legal options.

Where there is a sign with the guy in the wheelchair, it is usually yellow in the parking spot.

In the U.S. the guy in the wheel chair is on a blue sign and blue in the parking spot.