code: public static String color(RelationshipType rel) {
          if (rel == null) {
              return DFLT_EDGE_COLOR;
          }
          switch (rel) {
          case ACTS_IN:
              return "153,153,153";
          case HAS_COMPONENT:
              return "153,153,153";
          case HAS_MEMBER:
              return "153,153,153";
          case HAS_MODIFICATION:
              return "153,153,153";
          case HAS_PRODUCT:
              return "153,153,153";
          case HAS_VARIANT:
              return "153,153,153";
          case INCLUDES:
              return "153,153,153";
          case IS_A:
              return "153,153,153";
          case REACTANT_IN:
              return "153,153,153";
          case SUB_PROCESS_OF:
              return "153,153,153";
          case TRANSCRIBED_TO:
              return "153,153,153";
          case TRANSLATED_TO:
              return "153,153,153";
          case TRANSLOCATES:
              return "153,153,153";
          }
          return DFLT_EDGE_COLOR;
      }
docs: Returns an RGB tuple of the form {@code "x,x,x"} for the supplied
  {@link RelationshipType}. Defaults to black; RGB {@code "0,0,0"}.
  
  @param fe {@link RelationshipType}
  @return Non-null {@link String}