try use constant for a shader or tweak ambient diffuse speculat etc. If not helping, try see preferance of your computers video card, if not helps also you can do 2-d render pass and blend 2 copys of scene or add brightness param to your shader.
May be your color settings for your video card. I know my ati card and windows display preferences offer controls over these settings. I always make sure that it’s set to default as any tampering with color profiles has lead to unstable performance.