it might be the start of rust just have keep eye out on it try putting it in new soil it might not have space for the roots

Here is a link that might be useful: mint rust

I don't think it's rust, since I've never seen any orange and the only yellow I've seen is normal death when the water supply for the leaf dies - the leaf goes straight from green to brown/black. Replacing the soil would probably be a good idea anyways, though I wouldn't think the plant's grown enough for a bigger pot but plants have done more surprising things than grown roots quickly...

maybe a root rot.
they like well drained soil, not much fertilizer... like water but not being drawn . somehow I here a lot of herbs, in pots, kept inside ..get all kind of diseases. Mints are supposed to be some of the toughest and disease free herbs. Sometimes you cannot kill them if you wanted to..heh

Two things will do a mint plant rust and root root I seen other pic you got a tray under remove that I would add sand to compost that needed here (UK) as we get tons rain if in a hotter part it dry faster than here but I got 40+ mint plants and thinking getting more lol but all I done was ensure free flowing no blocks at bottom allow it out in sunny shaded area and mix a sand compost....

mint is good as if you cut it down to roots it will come back also if you split the plant you can have endless supply plants but worse you treat it better it will respond bit like thyme.....

Bit of care in lnk below

Here is a link that might be useful: mint dos and don'ts

It doesn't look like rust to me at all. That doesn't make the plant droop. There is some problem with the plant's ability to take up water.. This could be due to overwatering, underwatering or root rot as a consequence of overwatering. If it were mine I would remove it from its pot and divide it into two or three portions, each having some roots. Then I would repot in new medium and water well. If at all possible I'd place the divisions outdoors, not inside. After that I'd wait.

Mint roots, if healthy, are very tough. So here is what you can do to find out if in fact it is root rot: Cut most of the top and empty the pot, wash the roots under gentle water hose, or watering can.. Examine the roots. With a sharp pruner trim and/or get rid of all the rotten roots and corresponding tops. Repot the good healthy roots in a soil that is well drained.(can add some sand). in no time the healthy roots should sprout new growth.

If the mint is an indoor plant, can you move the pot outside to a sheltered location? Mother Nature is great medicine for ailing plants.

FataMorgana