You can freely change the color of individual POI actors unless their POI Type is set to POI Filter. The color for those type of POI actors is set according to their Availability status. For POI actors with the POI Type: POI, POI_Center; Click on the POI actor. From the Details menu, open Default > POI Info Struct. Click Color and select the desired color.   For POI actors with the POI Type: POI_Filter; Open the Construction Script of BP_POI. Find the Select function from the Image below and change its colors.