[mbbga1]

I've attached two renders one using MR's Physical Sky the other using a IBL background. The light source is in the distance behind the camera based on the environment settings. As you can see, the colors on the building appear more vivid when using the Sky option. Is there any way to improve the image lighting using the IBL option so the colors are more vivid?

[JTB]

Fist, it looks like your HDR image is over exposed... the sky color is much lighter than it should be, considering that it is a sunset image... so the colors are washed out.
Sometimes, the problem is the quality of HDRI, or the HDRI itself... if you want to have the look of the first image,then you can try another HDRI or keep the physical sky and use alpha channel to add a background in GIMP,PSP, or whatever you use.

[mbbga1]

Thanks for the feedback JTB. I went back and checked the MR file and found I had mapped the ambient lighting to match the HDR image which was causing the muted colors. I've turned these off and will re-render again when I get some time. I'll also try some of your suggestions. The HDR image is reasonably hi-res (about 1100x640). You're right about the over-exposure as it was the only way to bump up the lighting! Is there a way to set the direction of the sun onto the model (as with Physical Sky) yet use a HDR image instead so the model still receives sunlight and shadows as per the blue-sky render? Thanks.