Similar to
@AutoDoc Williams says, but I think I would put some shellac or other quick sealant on the higher parts and trying not to get it into the open grain where a red colour is wanted, you will be able to feel which direction you should be going in, very like which direction to plane in. Then you can use a red dye which shouldn’t affect the treated parts but should go into the open pores.
Then apply more shellac this time trying to get it into the pores to seal the red from the next stage which will be sanding the areas to be dyed black, and then doing so.
BUT please as was said, try it on scrap Ash first as this is just based on intuition.
Ian