A 'Group Build' is where a number of forum members each produce up to four models, in a choice of the popular scales, each one representing the theme of the particular GB. Each GB lasts four months, with a one month overlap between GB's, and are judged to provide a 1st, 2nd and 3rd place in each of the Categories, for Beginner, Intermediate and Advanced.
The full explanation and Rules, along with photo requirements, are shown in the general section of the GBs in the Modelling forums, as is a list of the forthcoming GB's, with dates.
Glenn, in addition to making notes, I often produce a colour swatch of paint mixes, using the mixed paint, and add notes to remind me what was used.
Here's the one I did recently, when matching 'mixed' Medium Sea Grey to the Model Master paint, using the Humbrol paints noted on the card.