Found this in the PDF documentation (
http://www.sparxsystems.com/bin/EAUserGuide.pdf)
Looks like what you're looking for but I haven't tried it myself.
SetAppearance (
long Scope,
long Item,
long Value)
Void Sets the visual appearance of the element.
Parameters:
Scope: Long - Scope of appearance set to
modify
1 – Base (Default appearance across entire
model)
To set appearance for the element (diagram
object) in a selected diagram only, see
Setting The Style in the DiagramObject
Class topic
Item: Long - Appearance feature to modify
0 – Background color
1 – Font Color
2 – Border Color
3 – Border Width
Value: Long - Value to set appearance to