since you're in a hurry, i'll be short

1 - fill it up more, this looks like dirt with some parts of grass

2 - add some lighter, more contrasty and yellower colours

3 - try to make some patches lighter and some darker

and lastly: here's something that might help ya

the grass on that squares is realistic enough imo for any sort of image.

I took the tiles into paint and put them together and they do look pretty realistic as is, although you could probably use a bit of a highlight color (but keep it dull) on the dirt, and a better shadow color than the reddish brown spots you have right now. The shadows in the dirt don't give it texture or anything at all, since they're few and far between and are hard to see even when zoomed in.

Looks like your edges aren't going to line up too nicely with the current patterning outlines of straight isometric, by the way. There's going to be that awkward line of 4 pixels at the top and bottom of each tiled piece and the empty block which is less noticable, but you may still want to fix it, on the left and right sides where the corners of three tiles would meet.