ICUMSA colour is based on the absorbancy of a sugar solution at pH 7.0 and filtered through a 0.45 micron mebrane. After filtartin part of colour compands in solution are retained on the filtration membrane. This color is not quantified as ICUMSA colour.
The colour contribution of these colourants in sugar solution and retained on o.45 micron membrane is referred as "hidden colour" (Godshall, 2005).
Technically, Hidden Colour is the contrubtion of colourants contained in a sugar solution filtered through a 0.8 micron membrane and retained on a 0,45 membrane (Godshall, 2005).
Godshall M.A., 2005, Understanding and controlling color development in mills and refineries, Proc. of S.I.T.