First, please clarify whatever question you may have. Since you don't appear to have one, I'll just decribe what causes this 'phenomenon.'
This thing that you are describing, if i am guessing right from your somewhat ambiguous explanation, is a basic function of your operating system. The window that has the blue bar on top is called the active window. There is absolutely nothing that you can do to change this, and anyway changing it would be about as bright as changing anything else in the 'c:\windows' folder. It will not actually happen "mostly in hotmail," it will happen in all windows.
The reason you might be experiencing these changes to your "top blue bar" could be from pop-ups or pop-unders causing a new window to open and "lightening your blue bar". Turn on pop-up blockers on your browser or download a toolbar with one on it. google toolbar works ok.
But yes, Damocles was right about the function of what you are experiencing. And since you are new to this you should stay clear of the folder Damocles mentioned.