Just because you can do something doesn't necessarily mean that you should do that thing. You could put white text on a white background on your website, but that doesn't mean that it's good for your visitors. If you have a story to tell, it's important that your visitors can actually read it, so you need to choose wisely in picking colors for the text and backgrounds on your website or in your desktop app. You could stick to safe but boring combinations of black and white, or you could guess what might be a good set of contrasting colors, or you could check out ColorSelector.
ColorSelector helps you to make sure that the text on your page can be read by your visitors. It tests your combinations not only for light and dark contrasts, but also keeps in mind color combinations that could get in the way of visitors with various types of color blindness. Enter color values directly as decimal- or hex values, use the built-in palettes, or the eyedropper tool can grab colors off the screen. Now everybody can see what you're talking about.
You can download ColorSelector for free. It's available for both Mac and Windows systems.