VAPOR GUI provides several ways to select colors in a Transfer Function. 

To change the color of an existing control point, click with the right mouse button over the control point.  Then choose "Edit Control Point".  Click the "Select" button to be provided with one or more color selectors from which you can choose the desired color.

To create a new color control point, Click with the right mouse button over the point in the color bar where you would like a new control point.  Choose "New Color Control Point" to produce a color control point having color interpolated from nearby color control points.  Then use the method above to reset the color to the desired value.

The DVR tab includes a color selector above the Transfer Function Editor.  To select the color at a color control point, first select the control point.  Then click in the color selector to choose the desired color.