Office only works with the RGB colours. If you're starting with CMYK colours, you'll need to do a CMYK>RGB conversion, for which you could use a function like:
Function CMYKtoRGB(C As Single, M As Single, Y As Single, K As Single) As String
Dim R As Long, G As Long, B As Long
C = (C * (1 - K) + K)
M = (M * (1 - K) + K)
Y = (Y * (1 - K) + K)
R = (1 - C) * 255
G = (1 - M) * 255
B = (1 - Y) * 255
CMYKtoRGB = "R: " & R & vbTab & "G: " & G & vbTab & "B: " & B
End Function
The following macro shows how the function might be used:
Sub Test()
MsgBox CMYKtoRGB(0, 0.498, 1, 0)
End Sub