Man, I can't imagine wearing that rig for "a couple of hours" in one sitting. What are you sanding on?
I wear a generic mask with replaceable cartridges and keep it sealed in a plastic bag when not in use.
I order mine from lockwood or homestead finishing products (they make transtint).
I have never seen wood dye in a store, maybe they have them at woodcraft.
You can probably use fabric dyes (rit) but they are not wood tones and probably fade easily.
Paint stores sometimes sell pigments/universal colorants. Artist's acrylics might be solulable in alcohol.
Most everything is solulable in acetone, so if unsure, mix it with acetone then add to your shellac.