I just mark them as Fire Red Culls with a picture. Not sure if you noticed, the above two pics were my culls in the internal breeder box. Look at the shrimp just below the rili, that shrimp was pretty red except maybe a small dot on the back that's a bit translucent. Don't be shy when culling, either cull hard or you'll cull long.
That makes me feel better about ruining 20 shrimp lives. lols