Alright, I'm not terribly experienced with tiles, but the point is the most important part-
1. The palette is very washed out, and whatever hue shifting there is is just neutral -> warm. The lightest colors are also at about half luminosity, that is to say not very light at all. You'll see that the darkest color for both edited tiles is a straight violet. You said you're new to pixel art so dramatic hue shifting isn't something you should expect to know just yet, but pixel art is more about implying features rather than actually drawing them out. It's the same principle with colors- a very cool color, even if it shouldn't fit with the palette (like that violet), implies very dark areas at such a small scale, and complementing the cool with warm highlights makes it much more visually appealing than a ramp of brown.
2. The clusters of dirt are basically in a 3x3 grid. Tiles are already at risk of being repetitive, and having a grid inside the grid adds way too much to the monotony.
3. Varied dirt clusters, but still tried to stay close to the original. There's not much to explain other than with the example.
4. Then you can try experimenting a bit, the one on the left adds more shadows and highlights. Like I said before, the complementary warm and cool are nice to look at, so having more warm and cool can make more of it look nice (in moderation, of course). The one on the right just has smaller dirt clusters to experiment with the pattern a bit more.
Not much else I can say, if one of the professionals wants to do a better job that would definitely be helpful, but at least I could clear up a few things.
