I see someone already replied while I was trying my hand at this. I'd pretty much say the same thing. Try to change the colour too give it more excitement, as well, lower the amount of colours you use, since you're really using more than you need to.

Here's my attempt.


As you can see, I went for more of a cartoony dirt colour using reds instead of going to from gray -> blue. I also pumped the saturation up quite a bit. Either way, hope you get some good advice from this thread, the actual shape is pretty well done in my opinion.

SUPER EDIT: My advice for colours would be to use higher saturation in general, you can lower the saturation as you go lighter and lighter with colours, as it's less noticable, but for darker colours, when you lower the saturation, it always seems to turn out boring. Also, hue shifting. For example starting with a light yellow -> dark red. This can be applied to other colours as well.