Mastulele

As a chemist I can certify that the alcohols used to dissolve shellac cannot be dried completely. The alcohol used in South Africa (and probably also in the USA) typically contain a minimum of 5 - 10% water. This can cause rusting of metal cans containing the shellac solution, fast. slow or slow depending on the can can coating used in the manufacture of the can.
Discolouration of the shellac solution is typical of polymerisation or oxidation of the shellac in the can. Discoloured shellac will not have the same properties as freshly dissolved shellac and the deeper the brown tint, the worse the properties are likely to be. Commercially made pre-dissolved shellac contains a stabiliser (anti-oxidant)which extends the shelf life considerably, but not indefinitely.
The best answer is to use only shellac which not discoloured not cloudy, or which has a sludge settled at the bottom of the can. Or to use a freshly made solution.