This example sets the current default color for magenta, then uses SetColorRGB to modify the magenta color.
Sub ModifyMagenta()
'Declare variables for color coponents
Dim magentaRed As Integer
Dim magentaGreen As Integer
Dim magentaBlue As Integer
With Session
'Restore the default colors
.RestoreDefaults rcColors
'set the defaults color values for magenta
magentaRed = 255
magentaGreen = 0
magentaBlue = 255
'Set plain text to black text on a magenta background
'And pause to view the results
.BackgndColor = rcMagenta
MsgBox "Default Magenta"
'Change the color components for magenta, decreasing the
'red value by 20 and using default green and blue values
.SetColorRGB rcMagenta, magentaRed - 20, magentaGreen, magentaBlue
MsgBox "Edited Magenta"
End With
End Sub