R_GetRGB() 
 Determines the composition of a specific colour
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 Syntax
        R_GetRGB ( nRegNr, @nRed, @nGreen, @nBlue )

 Parameters
        nRegNr          Number of the colour register of which the composition
                        has to be determined (range : 0-63)

        @nRed           Variable (BY REFERENCE !) to which the RED component
                        is returned (range : 0-63)

        @nGreen         Variable (BY REFERENCE !) to which the GREEN component
                        is returned (range : 0-63)

        @nBlue          Variable (BY REFERENCE !) to which the BLUE component
                        is returned (range : 0-63)

 Returns
        NIL             (The values are returned BY REFERENCE to the variables
                        nRed, nGreen and nBlue)

 Description
        R_GetRGB () determines the actual colour composition of a specific
        colour register. To determine the composition of the Clipper colours
        you may use the pre-defined register numbers in RPCXLib.CH.

 Example
        && Determine the components of the Clipper colour MAGENTA

        *-- RPCXLib definitions
        #include "RPCXLib.CH"

        LOCAL   nRed, nGreen, nBlue

        [...]

        R_GetRGB ( PL_MAGENTA, @nRed, @nGreen, @nBlue )

        ?'Magenta : RED ',nRed,' GREEN ',nGreen,' BLUE ',nBlue