A very nice feature in Blend is the fact that it supports multiple color systems. Per standard it's using RGB, which you can see in the properties view of any element containing some kind of ColorBrush. But actually Blend gives you the option of putting in CMYK, HLS and HSB too!
The way you can change between the different color modes is by clicking the underlined letters next to the colorpicker.
Note: If you choose CMYK, HLS or HSB colors, they get converted to a RGB color.
Remember to reuse the same color through out your app – do this by creating the color as a resource and then just reference it.
The easiest way to create a color as a resource is to click the tiny square next to the property and choose Convert to New Resource
Feel free to reach out to me if you need additional help or insight in working with RGB, HLS, HSB & CMYK in Blend or Visual Studio.