81dbfe30_essage_Sequence_Chart_Renderer__Meaning
[Source Attribute] arctextbgcolour, arctextbgcolor [Meaning] 
Only meaningful on a entity, this sets the default colour for the text background for all arc oriniationg from that entity. This can be overridden by textbgcolour attributes on specific arcs as desired.
Mscgen: A Message Sequence Chart Renderer
Meaning