css/less editor which shows colors

Try Crunch! - It's a LESS CSS editor that color codes, compiles and compresses. It's written in Adobe Air, so it's cross-platform. http://crunchapp.net/

Update - Visual Studio 2012 has a fantastic LESS editor, as a plug-in that you can install easily via the extensions manager. I believe that in VS2012, the extension is provided by Microsoft, and integrates with DotLess, and gives you a variety of options for when your LESS file is compiled (at compile, build or run times).

If you're interested in using Visual Studio, ReSharper 6 (in beta when that blog post was written, but released now) will show a thick underline in the specified color.

If you want something lighter-weight (or less expensive) than Visual Studio, the same folks who wrote ReSharper also make standalone IDEs. WebStorm 3 (currently in beta) shows color chips in the editor gutter. (Released versions may do this too; I can't find screenshots online.) Here's a screenshot I took from the beta; note the little gold square:

I use Coda:

http://panic.com/coda/

It can display colors when editing CSS.