How to use onchange together with two select lists in asp form

Hello, I have two select lists in the same asp page, one for choosing the product and one for choosing the color. I would like both select lists change without using submit button so that if the user choose Tshirt and the product shows Tshirt and then if the user choose red, the Tshirt the user has chosen is in red.

But currently I put these two select lists into two forms and using onchange with both of them. The problem is that after I first choose the Tshirt, it is ok. Then I start to choose the color, then the product changed at the same time to its default value but not the Tshirt I just chose.

How should I modify the code then ? Does any one has some idea? I have a reference page like

And I want the same function as in that page. Thanks very much in advance and hope I have made myself clear. Thanks again and wait for your reply!