One of the problems here is that since you are using a code-block within a Javascript file :
var v = document.getElementById("<%=ASPxFormLayout1_iva.ClientID%>");Javascript isn't going to know what your "<%= ... %>" blocks mean and thus will not be able to resolve this. This is going to leave you with the option of either explicitly passing the element into your ChangeComboColors function or hard-coding an ID so that you can resolve the proper element name. However, if you were to place this function within the ASPX page itself in a <script> tag, it's likely it would work as expected.