Hello, Ray_Newman..I was told by an old shooting friend,,long since gone to his reward..Don Metzger, to place one of those fluorescent orange posters sold at dollar store on angle behind target..when bullet goes thru..it looks like someone turned on an orange light thru hole..I bought the poster...But sadly the Michigan Schuetzen matches are just about gone so I never needed it at the short 100yd. range I now use.

If you hold a red camera filter and look through it into your spoting scope, it will help darken the holes in the red bull.

Frank

All of the above suggestions work - some better than others depending on weather conditions.

For our Military Matches, we use target frames with added supports behind the targets angled at approximately 30 degrees from vertical.  Large white plastic sheets are then stretched over these supports.  Because of the angle, the plastic sheets collect available light (even on overcast days) and reflect that light back through the bullet holes.  

The system isn't perfect as it can be seriously compromised by heavy mirage but it does work under all other conditions.

JackHughs