What kind of issue are you having? This works for me.
Activity.LoadLayout("main")
EditText1.Color = Colors.Magenta
As for assigning an image to the edit text, you can do that in code
Sub Globals
'These global variables will be redeclared each time the activity is created.
'These variables can only be accessed from this module.
Private EditText1 As EditText
Dim drw As BitmapDrawable
Dim bmp As Bitmap
End Sub
Sub Activity_Create(FirstTime As Boolean)
'Do not forget to load the layout file created with the visual designer. For example:
Activity.LoadLayout("main")
bmp.Initialize(File.DirAssets,"smiley.gif")
drw.Initialize(bmp)
EditText1.Background = drw
End Sub