From the appropriate Revit dialog, access the Windows Color dialog.
For example, in the Material Editor panel of the Material Browser, for the Graphics asset (Graphics tab), click the Color property value. (See Materials.)
In the Color dialog, select a color using one of the following methods:
Basic colors: A table of 48 commonly used colors is available. Click one of the boxes containing the desired color.
Hue, Sat, Lum, Red, Green, Blue boxes: Type values in these boxes to define the desired color.
Black and white settings box: Move the slider up and down to alter the amount of white or black in the color. The result displays in the color/solid box.
Custom colors: Add up to 16 custom colors. To add a custom color, click one of the 16 color squares under Custom Colors. Mix the new color by clicking one of the Basic Color squares or by choosing a PANTONE color. When you have finished mixing the color, click Add Custom.
When you have specified the desired color, click OK.