What is way to flash the screen WITHOUT using a Texture?

I was looking to flash the screen when a player takes a hit and I would like to flash only without using a Texture.

PRIMARY REASON :
If I use the texture then I have to reduce the alpha value on that texture which would then result in fillrate. As my target is mobile, I quietly want to do it without using a texture.

Your help is much appreicated.

Ramp the ambient lighting up from {normal} to pure white back to {normal} using RenderSettings.ambientLight

you can probably use an image effect to do this… since other than that any pixel you place on the screen is essentially a texture. but for Free users Textures is probably the only thing you can do.

http://docs.unity3d.com/Documentation/Components/script-GrayscaleEffect.html

this might be a start as far as image effects go