I wasn't able to figure out a solution using color. In the end, I used an asterisk in the label of the point instead of a color.
1.<telerik:LineSeriesDataFieldY="dataValue"Name="line1">2. <LabelsAppearance>3. <ClientTemplate>#if (dataItem.CorrAct.trim()) {# *#=dataItem.dataValue#* #} else {# #=dataItem.dataValue# #} #</ClientTemplate>4. </LabelsAppearance>5.</telerik:LineSeries>