In Internet Explorer 9 you will sometimes see differing colours in the address bar, the differing colours mean..

Colour

What it means

Red

The certificate is out of date, invalid, or has an error. For more information, see Certificate errors: frequently asked questions.

Yellow

The authenticity of the certificate or certification authority that issued it can't be verified. This might indicate a problem with the certification authority's website.

White

The certificate has normal validation. This means that communication with the website is encrypted.

Green

The certificate uses extended validation. This means that communication between your browser and website is encrypted and that the certification authority has confirmed the website is owned or operated by a business that is legally organized under the jurisdiction shown in the certificate and on the Security Status bar.

Read more tips for making secure online transactions in Internet Explorer 9

also note the warning on encrypted connections:

An encrypted connection doesn't guarantee that the website is trustworthy. Your privacy can still be compromised by the way the website uses or distributes your information.”

Rob