How to remove red stains from my dogs beard?

I have an Old English Sheepdog that has red staining on his beard. I use filtered water in stainless steel or ceramic bowls for the water.

I feed him a high quality grain-free dry dog food with NO artificial coloring or additives.

I have tried to mildly bleach his beard with diluted hydrogen peroxide or lemon juice, but it doesn't stay there long because he promptly licks it off and starts grooming himself to get rid of the yucky taste!

I have tried to use white (and apple cider) vinegar in his water, but he is very reluctant to drink it because it smells vinegary.

I don't want to mask the beard hair with bluing or chalk or corn starch, as the red staining is just covered up.

I would rather not give him Angels Eyes as a food additive, as he doesn't need a low-level dose of anti-biotics all of the time, making for resistant strains later on......

What is the best method to remove (and continue to have no) red stains on a dogs beard?

No, clipping beard is NOT an option!!

I don't want to mask the stain with anything, I just want the staining to vanish!!

Thanks for your help!