After tracing it back through the source, it does seem to flag the item properly (all the way back in Selection.cs). Are you refreshing the plot after you select the item? (InvalidatePlot(false) on the view or model). The mouse event will change the flag but it won't actually display anything on it's own unless you tell the view to update manually.
Also imo at least selection colour should be global to the model itself, not series. I can't really think of a reason you'd want multiple different selection colours unless you're selecting different items for different actions.
Also imo at least selection colour should be global to the model itself, not series. I can't really think of a reason you'd want multiple different selection colours unless you're selecting different items for different actions.