Dim myElem As Element
Dim myScrn As DynPicture
Set myScrn = thisProject.DynPictures.Item("a")
If Not myScrn Is Nothing Then
Set myElem = myScrn.Elements.Item("aa")
If Not myElem Is Nothing Then
myElem.BackColor = RGB(89, 58, 12)
You can use the UpdateAll method to redraw the sreen. However this change will only last while the screen is open. After a reopen the button will show it's original color again.
Generally I would recommend to use the dynamic background color through a variable, which can be configured in the editor, instead of doing this by VBA in the runtime.