Looks good so far. A little bit different things (styles) thrown together and the purple, red and even the green are a little too over saturated. Otherwise pretty nice.
as christoph said, its a bit of a mix and match. you have an isometric block, with a flat 2d background, the block is a pastel sort of color, and the background is overly bright. If i was doing it, and the game has isometric objects, i would make the background objects isometric as well. The buttons isometric, or white buttons with a bit of transparency. in a isometric shape. but you can't have white buttons cause of the background, or rotate your buttons 45 degrees, see how that looks. But it all depends on your game field design,
How does it look now ? The white cube is starting platform on which other cubes with different colours will get stacked I have changed the buttons and trees to isometric Any more suggestions and improvements would be highly appreciated
Looks great! Is the orange icon with the cube on it what needs to be tapped to start? If not, then maybe you can put a pulsing finger (like it's tapping) that grows and shrinks in the middle of the screen to signal tap to start. That way, people who don't read English will know what to do immediately, but that's just a suggestion in case you are interested in implementing it. Otherwise great job. Looks nice.
Oops. I responded to your first post. I see that there was another UI posted later. I was commenting on the first design. The 2nd design's 'tap to play is more obvious so that's an improvement. Looks good