Nothing wrong with it. Maybe it rained earlier and the dirt got washed off the tank?
However, you'll never get better at weathering if you don't try it

A simple way to get a tank to look dirty is to put a wash of dirt-coloured paint on it. Just take some acrylic paint, put it in a palette and thin it until it flows like water, then brush it all over the model. Make it thicker on the lower hull and over the suspension, thinner on top — with which I mean to use less water in the wash for the lower parts and more water for the upper ones.
If the wash covers too well, obliterating the paint where you don't want it to, brush plain water straight onto the model there (before the wash sets, of course). Another thing to look out for is pooling of the wash at the bottom of (near-)vertical surfaces, which you can wick away with your brush before it dries (probably repeatedly). If you don't, you'll get a horizontal line of "dirt" there which will look unrealistic.
The result of this will be a dusty/dirty colour over the paint, and especially in grooves and around detail, where dirt would accumulate on the real thing.
A second step can then be to remove some of the dirt again where it might rub off on the real tank, for example because the crewmen walk or sit there, grab handles, open hatches, etc. You can do this by drybrushing there with the colour of the paint underneath the wash.
Real tanks get
very dirty very quickly, so you can't really overdo this kind of weathering. The only thing you'll want to avoid, IMHO, is to totally obliterate the paint underneath all over the model

Well, that, and add paint chips and tons of rust — in the real world, all these vehicles were so new that chipped paint and/or rust anywhere would be almost always unrealistic.