With any other app, yes. It is as easy as right-clicking the app>Show package contents>Contents>Resources and dropping the new icon in there. With skype, this used to work on older builds of Skype 2.8. But with the new versions, Skype does an integrity check. If anything is modified, it won't work. There are a bunch of threads on Skype's forums trying to get the devs to give us the option between a color/monochrome menubar icon. No one is listening, so it's impossible for now. You can use other instant messengers like Adium that have Skype API. Problem is you need Skype opened, Adium opened and the menubar icon turned off in Skype. It's too much work for an aesthetic issue.
There is also something called
SkypeMenuX. I've never tried it, but it looks like it might change the icon for you. Again, it's too much work for something minor. Do what I did and voice your opinion about a monochrome icon on Skype's forums.