'за декларируйте тип и API-шку
Private Type ChooseColor
  lStructSize As Long
  hwndOwner As Long
  hInstance As Long
  rgbResult As Long
  lpCustColors As String
  flags As Long
  lCustData As Long
  lpfnHook As Long
  lpTemplateName As String
End Type

Private Declare Function ChooseColor Lib "comdlg32.dll" Alias "ChooseColorA" (pChoosecolor As ChooseColor) As Long

'вставьте код для вызова диалогового окна
Dim cc As ChooseColor
Dim CustColor(16) As Long
cc.lStructSize = Len(cc)
cc.hwndOwner = Form1.hWnd
cc.hInstance = App.hInstance
cc.flags = 0
cc.lpCustColors = String$(16 * 4, 0)
Dim a
Dim x
Dim c1
Dim c2
Dim c3
Dim c4
a = ChooseColor(cc)
Cls
If (a) Then
  MsgBox "Color chosen:" & Str$(cc.rgbResult)

        For x = 1 To Len(cc.lpCustColors) Step 4
            c1 = Asc(Mid$(cc.lpCustColors, x, 1))
            c2 = Asc(Mid$(cc.lpCustColors, x + 1, 1))
            c3 = Asc(Mid$(cc.lpCustColors, x + 2, 1))
            c4 = Asc(Mid$(cc.lpCustColors, x + 3, 1))
            CustColor(x / 4) = (c1) + (c2 * 256) + (c3 * 65536) + (c4 * 16777216)
            MsgBox "Custom Color " & Int(x / 4) & " = " & CustColor(x / 4)
        Next x
Else
        MsgBox "Cancel was pressed"
End If