Fix #2533139 - Color tests failing in older IE's Edit

regex.exec() was returning values of (empty String) instead of undefined in IE.

This fix corrects some failing tests in older IE's where they appear to be returning an empty string instead of undefined.

http://yuilibrary.com/projects/yui3/ticket/2533139

Also address another IE bug where `str[pos]` was used for character lookup but this is not supported in older versions of IE. `str[pos]` was replaced with `str.charAt(pos)`

All tests are now passing in IE6, 7, and 8.

@davglass