I have a wizard application for MS Windows which is a typical Next-Next-Finish type. The application has a default vendor specific window background theme which persists through all the windows of the application (i.e Vendor logo and background color for each screen). I want to develop a similar application wizard for Mac. I noticed that all the applications have a default Grey colored window in Mac. Is there any design guideline provided by Apple which states that the background color of the windows must be the default? Can I use custom colors and images for the application? Any ideas will be highly appreciated.
The problem of using custom colors for windows/controls in a Windows application is that they don't change according to the theme chosen. The ControlColor/ControlText and other choices change as per the theme in use. I think it must be the same for Macs. Not sure though... Hope you get clues from other answers...
Apple has a pretty comprehensive document called the Apple Human Interface Guidelines. Most Mac OS X users expect applications to look and behave according to these guidelines and are (usually) slightly less forgiving of applications that deviate from these. It is, of course, completely possible to customize your application however you wish, but I would strongly suggest reading the document (link is in the name) before deciding how you want to proceed.
They are just guidelines, meaning that if you have a good reason for not following them, it's not the end of the world, but once you familiarize yourself with them, you're more likely to be able to blend your customizations into something that doesn't feel totally foreign to your Mac user base. Even if your primary user base is recent Windows converts, this will have the effect of giving them something that they are mostly familiar with already and allowing them to start a transition into the design expectations of other Mac applications.
In the end, though, it's completely up to you. You also may want to take a look at how Apple handles these assistant wizards. Look at the installer application (many products customize this, and if you have the tools installed, you can see where it's easily customizable to get some clues as what you might allow to be customized in your application). Also, you can take a look at some of the "wizards" that Xcode uses and that the Apple Pro packages use (if you have access to them) or that various iLife applications use (you should have access to those). That research combined with at least perusing the Apple Human Interface Guidelines should get you well on your way to something that satisfies your current customers, your boss (if you have one ;)) and any customers that are already entrenched Mac OS X users...
You don't have to make your background gray, but you'd better have a darned good reason (and a professional designer) for doing otherwise. Mac users generally have an idea of what a Mac application ought to look like, and designs that throw up lots of odd colors and custom graphics tend to feel "wrong." Just like any field, you can break the rules and get away with it, but you really need to know what you're doing or you'll just wind up looking foolish.
My advice as a longtime Mac user is to prefer consistency with the Mac UI over consistency with the Windows version of your program. It gives a better user experience. Like Jason Coco said, take a look at the wizards included with Mac OS X — the Mac term is "Assistants" and several are included in the folder /Applications/Utilities. Installer is also this sort of program.