Is it possible to calculate chromacity coordinates using absorbance values?
This is a partial answer to your question.  I have used chromaticity coordinates for various scientific endeavours (air quality mainly).  I found that it is a very powerful means of determining colour differences.

A bit of searching and there is somewhat of a precedent for using chromaticity for blood plasma.  An example is the article Features of Color Characteristics of Blood Plasma of Patients with Liver Cirrhosis in the Colorimetric System XYZ as Compared with Healthy People (Dolomatov et al. 2012) - even though it may be a different context to your study, but does show that it can and has been done before.  Specifically, their method was


  by colorimetric method, based on absorption electromagnetic radiation in visible area.


There are several more examples of how it can be used.