Calculating the Presence of Anthocyanin in a Solution
It is not a trivial exercise because it is not a single compound in red cabbage which causes this color (anthocyanin is a class not a single compound). There is a whole master's thesis dedicated for isolating red cabbage pigments from the University of Ohio. https://etd.ohiolink.edu/!etd.send_file?accession=osu1437655932&amp;disposition=inline

One can determine monomeric anthocyanin using UV-Vis spectrophotometry by measuring absorbances at two wavelengths. It is always good to mention what apparatus your school has.

Giusti MM, Wrolstad RE. 1996. Characterization of red radish anthocyanins. J Food Sci 61(2):322-6.

Giusti MM and Wrolstad RE. 2005. Characterization and measurement of anthocyanins by UVvisible spectroscopy. In: Handbook of Food Analytical Chemistry. RE Wrolstad, SJ Schwartz (eds). John Wiley &amp; Sons Inc. New York .p: F1.2.1-F1.2.13