What can I use to paint the chloaca of a hen to mark her eggs?

[as110]

I have 2 hens that finished quarantine and now I have to toss their eggs. I want to put them back into the coop and I need to know which eggs need to be tossed.
What can I use to paint the backside of these hens so their eggs will get stained? That would help to know which eggs are bad. I didn't treat all of the flock because they were all healthy looking and I didn't want to throw away 70+ eggs just to be safe.
2 hens' eggs are not too much to waste, we can handle that if there is a way to separate their eggs from the rest.

 

[WVduckchick]

I have heard of using food coloring, squeeze a little up their vent, but I've never done it.

What did you give them that you need to toss the eggs?

 

[as110]

I have heard of using food coloring, squeeze a little up their vent, but I've never done it.

What did you give them that you need to toss the eggs?

tetracycline
the package says not to slaughter food animals for 5 days and a thread here said to throw the eggs away. While they were on the med I fried their eggs and fed them back to the sick hens so the medication was no wasted.
I had enough of keeping sick hens in the basement lol.