Where are you getting that from? I can seem to use a UDF on every row (65536 cells).there seems to be a limit in Excel for the maximum number of UDFs on one sheet, only about 32704 cells can have a UDF
Public Function XYZ2Yxy(CIE_XYZ)
'Colorspace conversion, from XYZ to Yxy
Dim X, Y, Z, Sum As Double
Dim Yxy(1 To 3) As Variant
X = CIE_XYZ(1)
Y = CIE_XYZ(2)
Z = CIE_XYZ(3)
Sum = X + Y + Z
If Sum = 0 Then Sum = 1E-99 'hmmm
Yxy(1) = Y
Yxy(2) = X / Sum
Yxy(3) = Y / Sum
XYZ2Yxy = Yxy
End Function