How can we change the color of a custom button?(See description)

This can be pulled off, but you need to get crafty here's one I prepared with pink candy polka dots:

1. go to Setup > Create > Objects > [Your Object] > Custom Buttons and Links, and hit New,
2. create a Detail Page Button whose behaviour is to Execute JavaScript,
3. modify the following JavaScript, using your lowercased button name in lieu of your_button:
   (function() {
   var button = document.getElementsByName('your_button')[0];
   button.style.background = 'url("https://addons.mozilla.org/_files/311357/preview_small.jpg")';
   }());
4. use a base64 encoder to format your JavaScript, which we'll load and execute as a Data URI,
5. paste it into the formula, noting the data:application/javascript;base64, prefix:

{!REQUIRESCRIPT("data:application/javascript;base64,KGZ1bmN0aW9...wZyIpJzsKfSgpKTs=")}
window.location = '/apex/ns__PageName'; //button action when clicked

The usual caveat applies: using a Data URI here may be subject to future validation, but rest assured for now Salesforce support the use of REQUIRESCRIPT in a Custom Button. Your use is only cosmetic.

Further, if you wanted to separate the button click outcome from the pink candy polka dots, you could put the cosmetic JavaScript in a separate Custom Link which you place somewhere on the page layout. It's one novel way of running JavaScript in the scope of a standard page without needing to configure the sidebar or use a narrow Home Page Component.