How to set color of a button using it's ID?

First of all, you need to know how to use ClientID with propery ClientIDMode attributes to get the values translated to HTML id attributes the way you want. If you are not clear about this topic, please read:
http://weblogs.asp.net/asptest/archive/2009/01/06/asp-net-4-0-clientid-overview.aspx[^],
http://msdn.microsoft.com/en-us/library/system.web.ui.control.clientid.aspx[^].

The rest is on pure HTML and JavaScript. You need to use the value of the HTML id attribute, to find the element in question.

For the first step, you can create an optional CSS style which you can dynamically add or remove. For purpose of the illustration, let's define two colors, normal and highlight, for example:

.normal { background-color: silver; }
.highlight { background-color: yellow; }

In JavaScript, you can find the button by ID and change the color by adding, removing or changing the style class in response to some event. The best way to do it I could advise would be using jQuery. Again, just for some working example, let's consider the case when we change the colors when mouse goes on and out of the button:

$(document).ready(function() {
    myButton = $("#someId"); // "#..." is the ID descriptor
    myButton.hover(
        function() { // mouse goes in:
           $(this).removeClass("normal");
           $(this).addClass("highlight");
        }, function() { // mouse goes out:
           $(this).removeClass("highlight");
           $(this).addClass("normal");
    });
}

Please see:
http://api.jquery.com/category/selectors/[^],
http://api.jquery.com/id-selector/[^],
http://api.jquery.com/addClass/[^],
http://api.jquery.com/removeClass/[^],
http://api.jquery.com/hover/[^].

If you need to learn jQuery (highly recommended), please see:
http://en.wikipedia.org/wiki/JQuery[^],
http://jquery.com/[^],
http://learn.jquery.com/[^],
http://learn.jquery.com/using-jquery-core/[^],
http://learn.jquery.com/about-jquery/how-jquery-works/[^] (start from here).

—SA