Went back to not using a special dummy BugMarker, because it turned out to be less versatile.

Materials/bug_marker.gdshader

@@ -3,7 +3,7 @@
 shader_type spatial;
 render_mode blend_mix,depth_test_disabled,cull_disabled,unshaded;
 
-uniform vec3 colour : source_color = vec3(1.0f, 0.0f, 0.0f);
+uniform vec3 colour : source_color = vec3(1.0f);
 uniform sampler2D msdf_icon : filter_linear_mipmap,hint_default_black;
 uniform float shape_thickness : hint_range(0.0f,1.0f,0.01) = 0.95f;
 uniform vec3 line_colour : source_color = vec3(0.0f);