AJAX :: Change Textbox Backcolor In An Updatepanel On Button Click?

I'm using a searchoption on my website. (language = VB)

The user can choose a searchcriteria out of a dropdownlist (Artist name, Description, Price, Genre, Type) . If the user selects one of the options in the dropdownlist the correct textboxes are shown.

For example: if the user selects Price he gets two textboxes (one for the min price and one for the max price) if the user selects Artist Name he gets one textbox

The dropdownlist and the textboxes are put into an updatepanel. The searchbutton is outside the updatepanel.

Now for the problem:

If the user doesn't fill in a textbox and presses the searchbutton, the backcolor of that empty textbox should be turned red. The searchbutton is set as a trigger and the panel is updated on the buttonclick, but the colors don't show.

In the clickevent of the button I've said the following code:

If txtCriteria.text = "" then
txtCriteria.backcolor = Red
Else
txtCriteria.backcolor = White
End if

Quite a simple piece of code. This works if I remove the updatepanel from my page, but I'd like to keep the updatepanel because it's really nice :p.