Huh, the photo you just posted of Mini mum had me remembering this picture I took while on a study abroad in Thailand a while back, a little guy ID'd as Microhyla heymonsi by the folks on iNaturalist.
And now I'm just curious. Is that "brown on top, black on the sides/bottom" look just common for small frogs? Or are these genuses related ('Micro' and 'Mini' does give that vibe, lmao)?
So Microhyla heymonsi and Mini mum are in the same family, Microhylidae, but that does not explain the colour pattern because this kind of flank stripe occurs in various other families, and is also lacking in many other microhylids. The real explanation is that there basically don't seem to be that many different colour programmes that frogs can run, and one of them is a bold colour contrast like this along the flank. Several bufonids, for instance, which belong to a very different group of frogs (bufonid toads and hylid tree frogs are actually moderately closely related, whereas microhylids are more closely related to e.g. ranid and rhacophorid frogs than to that group) frequently exhibit a similar colour pattern.
