From earliest days, Interest Badges were part of the Brownie programme. They were optional extras for Brownies to work on in their free time. And prior to 1968, they could only be done by Brownies who had passed their Golden Hand badge - meaning most girls had little or no time in which to earn them before moving up to Guides.
Originally, the bades were split into four categories, and were made of coloured felt, with matching-coloured embroidery. They matched the Wolf Cub badges of the same era, other than that the Athlete figure wore a skirt rather than shorts.
The four categories were: Intelligence (blue), Handcraft (yellow), Service to Others (red) and Physical Health (green).
In the 1930s a change was made, initially to all the badges being on brown felt, as the colour-linked categories were done away with. Some appear with lettering, as well as some without. They continued into World War 2.
After World War 2 and into the early 1990s were the most familiar style of Brownie interest badges, the woven brown triangle. This is partly because, from 1968 onwards, there was no restriction on when Interest Badges could be done, meaning that Brownies could do the badges at any time during their three years membership.
In the 1990s all of the previous badges came to an end, and a new set of interest badges was introduced, as rhombus shapes. The standard badges were in yellow with blue embroidery around the edges, with the Advanced stages some badges had being in blue with blue embroidery.