Palette with many holes
I find 000 brushes most useful. 00000 brush less useful but nevertheless essential. Buy good to great brushes - no bargain brands, please. It is absolutely essential that tip of the brush holds together. Also, be very careful and use these brushes only for spotting.
What I do is, a day before spotting, I put a small amount of (a drop or two) dye on two places in the palette. Then add few drops of water. Let it dry over night. It spreads and makes a patch of very thin coat of dye. Next day, I wet my brush very slightly and dab on the dried area to pick up very small amount of dye. I do some trial "spotting" on a sheet of paper (I find envelopes junk mails come in great for this purpose!) until the density is little less than just right. Then spot the print. Repeat this process until I'm satisfied that spot no longer attracts attention. The density needs to be built up by repeated application. This method is so much easier than diluting the dye to match the tone.
The goal is NOT to paint the fault entirely or it has to match exactly. You'll be amazed, a dot in the middle of white area takes attention away from the blotch. If you stand a foot or two away from the print and you can't find it, you are DONE. If your dot is too strong or too dark, it will attract attention, so it is essential to use small dots and less density.
Next time you go to a museum, examine prints very closely.... you'll be amazed how "imperfect" of a spotting job great prints have.... yet, at distance, they blend in. That's all we need.
It's a lot easier than it sounds. If you make a mistake, just wash the print and dry, and start over again. You'll get a hang of it after few times.