The idea here is to use colour and other graphic tools to clarify calculations in which the individual parts of the result come from a different part of the structure. Clicking on a number shows/highlights that individual part of the calculation.
The red and blue parts highlight the contributions of the first and second matrix to the product; the purple part shows the result of that part of the calculation.
Here's the original version of this widget:
There's a misuse of colour in this version. Colour is a focusing element in visual design; colour on a predominantly black and white diagram focuses attention on the coloured parts. But in the definition of minors and cofactors, the numbers under the coloured bars are actually discarded; the minors and cofactors are made up of the remaining numbers. A better use of colour would be to make the bars gray (which signifies "not active" or something similar in current onscreen language) and to make them partially obliterate the numbers they cover. Thus:
Colour is used correctly here is to focus attention on the + or - sign (on the right side of the widget).