So always use a jar with a screw top and put the lid back on the moment you stop for any length of time. You can also use metho for dissolving the dye, you might find it cheaper.

However, even spirit-based dye will have a hard time penetrating dried shellac. If it does, then it's because you've not put enough shellac on to seal the wood. That's why it's called sealer, as it should prevent the wood absorbing anything else put on top of it. Stain first, then seal. Never the other way round. You can paint over sealer, but not stain properly. Any stain that may takes will probably be sitting on the sealer, and not in the wood.