also the pigments in the scented oils used to massage the deities can cause this.

Yes most often this discolorsation is caused by bathing.

The usual method for removing stains from marble is Hydrogen Peroxide or Borax. However, as both these are products which have nasty reactions on skin.. I don't think they would be suitable for using on the lord!

Here is a tip I found on a marble kitchen top manufacturers' website:
Use a paper towel to mop up the excess from any spill that occurs. Once you have done so, lightly dust the stained area with an absorbent like cornstarch. Leave it standing for an hour or two. Next, wipe the stain gently with a soft, damp cloth and towel the area dry.

so maybe you could try wiping and using the cornstarch after you bathe?

I guess the other question is how OFTEN do you bathe?
perhaps if te problem is getting to a particularly bad state you could do bathing en Manah and just do physical bathing for special occasions (as done with JBS deities)

Your servant
Radhesyam dd