1. So that any colour change can be considered. The colour for a positive result is similar to the normal colour. (I think)
2.Monosaccharides and disaccharides are not soluble in alcohol. Starch, nucleic acids and proteins are insoluble in alcohol. In order for the emulsion test to work, the sample must be soluble in alcohol, like lipids. Water is then added and the solution shaken, resulting in a white emulsion layer above the water.
3.Test various concs of lipids and place an equal volume of each emulsion solution into a colorimeter, testing it against absorbtion and transmission.