added support for auto color with _note and _par

src/participant.typ

@@ -13,6 +13,7 @@
   "queue",
   "custom"
 )
+#let DEFAULT-COLOR = rgb("#E2E2F0")
 
 #let _par(
   name,
@@ -20,11 +21,14 @@
   from-start: true,
   invisible: false,
   shape: "participant",
-  color: rgb("#E2E2F0"),
+  color: DEFAULT-COLOR,
   custom-image: none,
   show-bottom: true,
   show-top: true,
 ) = {
+  if color == auto {
+    color = DEFAULT-COLOR
+  }
   return ((
     type: "par",
     name: name,