With Vectorworks 2008, there was huge change to the color palette. From now on we are not limited to 256 colors, and you can arrange colors in palettes to make them easier to find.
To create your own custom color, you just need the color information (CMYK, or RGB numbers) and the attributes palette.
Go to the Attributes Palette.
Click on the Solid Color Fill part of the palette, just under the Fill style button.
Click on the Standard Color Picker button, the first button.