Mine looks like a dryer vent. Can you post a photo?

You can get any kind of exterior duct cover from white plastic to craftsman copper - you might want to consider getting a more expensive one than the builder gave you? Or is your issue with the actual quality of the installation?

Here's mine:

Paint it.
If you go looking for something differant be sure it is made specifically for range hoods. The wall caps for those have very light wt aluminum flaps, anything heavier will cut down efficiency dramatically. If you do manage to find something post it here, I have never found a suitable alternative.

It was dark when I got home so I couldn't take a picture.
It's metal and kind of big, not as subtle as annkh's. Also, it's not really low down like our dryer vent, nor up near the roof. It's kind of in the middle of the back side of our house, so DH hates it. He says it looks like the vent he's see on the back of a diner or something.

Here is the vent from the OTR Microwave exhaust:

{{!gwi}}

In contrast, here is the more subtle dryer vent, which is close to the ground (it looks the same size because I zoomed in closer, but it's actually much smaller):

{{!gwi}}

Can you take photo back further? No one would be standing that close to your house turned to the wall.

Can you find another close to the color of your house/or paint it so it would blend in?

Here is ours (the larger one in this picture). It seems about the same size as yours since it vertically takes up about 2 full planks. Mine is also in the middle of the side of the house, a little above the top of our kitchen window (window pictured is a bathroom window, but it is around that same height). That's a straight run to the side of the house from the top of my range vent. I'd imagine you'd want that run as straight as possible for efficiency sake, rather than running it down to ground level or up to roof level.

I think if you paint it to match your siding it'll just be one of those things you don't notice after a bit. I couldn't even have told you what mine looked like before I went out to snap the picture this morning, let alone that there are 3 little ones nearby (after thinking on it, I guess those are exhaust spots for 2 bathrooms and the dryer).

Here's ours. Looks very much like yours. They covered it with a piece of aluminum siding to match the house & we still have to paint the wood piece surrounding it. Ours is also a straight run from the top of the range vent.
We were also a little take aback when we first saw it but I don't notice it now. Your post reminded me that it still need to be painted.