Rainbow flashes could be a sign of retinal damage or retinal detachment. An immediate visit to your eye doctor is called for.
If you have had diabetes for a long period of time you should get an annual diabetic retina screen to check for retinopathy. Diabetic retinopathy has no symptom until damage to the eyes is severe. As diabetes86 stated, call your Optometrist.
Flashes of any kind means you should call your optometrist NOW.