iMac display changes?

[AdamA9]

Guys, I have an iMac 21.5" and when I start it up the picture is clear, crisp and bright for about 2 seconds before it dims. I've searched and cannot find an auto dimming setting, neither can I see why the colours would look duller than when it's first booting up.

Does anyone know why it does this, and how I can keep the original brightness from when it first starts?

Thanks.

[spinnerlys]

System Preferences > Energy Saver > Automatically reduce brightness before display goes to sleep or System Preferences > Displays > Color > Color Profile.