I'm using the correct code (there's no actual code in part 1's
QuerySchemeColor).

I thnk I'm calling HSLtoRGB correctly, with 4 arguments:  

HSLtoRGB RGB(79,129,189),0,0,.25

Why would "RGB(79,129,189)" return 0?  Shouldn't RGB(79,129,189) return the
long RGB value of "R79, G129, B189"?  

In any case below is a copy of the code I'm using.  The only modifications
I've made to your posted code are as follows:

In Colours2 I added "Call QuerySchemeColor(HexString)" to Case FF and
commented out the messagebox line in that case.  That added line is a copy
and paste of your code at Case D4 To DF.

In QuerySchemeColour I added the following lines at the end.  I only did
this because it seems that HSLtoRGB isn't recognising the function RGB as a
valid first argument - I figured I'd try passing a long value as the first
argument to see if I got any different results.

Dim lngr As Long
Dim lngg As Long
Dim lngb As Long
Dim lngrgb As Long

lngr = CLng("&H" & Mid$(ThemeColorHex, 7, 2))
lngg = CLng("&H" & Mid$(ThemeColorHex, 5, 2))
lngb = CLng("&H" & Mid$(ThemeColorHex, 3, 2))
lngrgb = RGB(lngr, lngg, lngb)

HSLtoRGB lngrgb, 0, 0, 0.25
<<

In HSLtoRGB I added "MsgBox RGB" to the end to display the RGB value that
that macro ends up with.  (Can I make a suggestion?  Isn't RGB the name of a
VB function? You might want to change the name of that variable.)

No matter what selection I run Colours2 from, HSLtoRGB returns "4210752".

From the following code can you see what I'm doing wrong?

Thanks again, code follows...

Sub Colours2()

   Dim HexString As String
   HexString = Right$(String$(7, "0") & Hex$(Selection.Font.Color), 8)

   Select Case Left$(HexString, 2)

       Case "00"
           If HexString = "00C8C67F" Then
               MsgBox "The colour of the Selection is not all the same"
           Else
               MsgBox "The colour is an RGB value:" & vbCr & _
                      "Red: " & _
                          CLng("&H" & Mid$(HexString, 7, 2)) & vbCr & _
                      "Green: " & _
                          CLng("&H" & Mid$(HexString, 5, 2)) & vbCr & _
                      "Blue: " & _
                          CLng("&H" & Mid$(HexString, 3, 2))
           End If

       Case "FF"
'ADDED CODE
           Call QuerySchemeColor(HexString)
'END ADDED CODE
           'MsgBox "The colour is set to the default of 'Automatic'"

       Case "80"
           MsgBox "The colour is an OLE Color"

       Case "D4" To "DF"
           Call QuerySchemeColor(HexString)

       Case Else
           MsgBox "The colour is of an unknown type." & vbCr & _
                  "The code is: 0x" & HexString

   End Select
End Sub

Sub QuerySchemeColor(HexString As String)

   Dim SchemeColorByte As String
   Dim ZeroByte        As String
   Dim DarknessByte    As String
   Dim LightnessByte   As String

   Dim SchemeColor As Long
   Dim Darkness    As Long
   Dim Lightness   As Long

   Dim SchemeColorName   As String
   Dim TintingAndShading As String

   SchemeColorByte = Mid$(HexString, 1, 2)
   ZeroByte = Mid$(HexString, 3, 2)
   DarknessByte = Mid$(HexString, 5, 2)
   LightnessByte = Mid$(HexString, 7, 2)

   SchemeColor = "&H" & Right$(SchemeColorByte, 1)

   ' New variables
   Dim ThemeColor     As MsoThemeColorSchemeIndex
   Dim ThemeColorRGB  As Long
   Dim ThemeColorHex  As String

   ' Changed translation, now to theme colour scheme index
   Select Case SchemeColor
       Case wdThemeColorMainDark1:     ThemeColor = msoThemeDark1
       Case wdThemeColorMainLight1:    ThemeColor = msoThemeLight1
       Case wdThemeColorMainDark2:     ThemeColor = msoThemeDark2
       Case wdThemeColorMainLight2:    ThemeColor = msoThemeLight2
       Case wdThemeColorAccent1:       ThemeColor = msoThemeAccent1
       Case wdThemeColorAccent2:       ThemeColor = msoThemeAccent2
       Case wdThemeColorAccent3:       ThemeColor = msoThemeAccent3
       Case wdThemeColorAccent4:       ThemeColor = msoThemeAccent4
       Case wdThemeColorAccent5:       ThemeColor = msoThemeAccent5
       Case wdThemeColorAccent6:       ThemeColor = msoThemeAccent6
       Case wdThemeColorHyperlink:     ThemeColor = msoThemeHyperlink
       Case wdThemeColorHyperlinkFollowed:
                            ThemeColor = msoThemeFollowedHyperlink
       Case wdThemeColorBackground1:   ThemeColor = msoThemeLight1
       Case wdThemeColorText1:         ThemeColor = msoThemeDark1
       Case wdThemeColorBackground2:   ThemeColor = msoThemeLight2
       Case wdThemeColorText2:         ThemeColor = msoThemeDark2
       Case Else:           ' This shouldn't really ever happen
   End Select

   ' Pick up the RGB and translate to a hex string
   ThemeColorRGB = _
       ActiveDocument.DocumentTheme.ThemeColorScheme(ThemeColor).RGB
   ThemeColorHex = Right$(String$(7, "0") & Hex$(ThemeColorRGB), 8)

   Lightness = 100 - ("&H" & LightnessByte) / &HFF * 100
   Darkness = 100 - ("&H" & DarknessByte) / &HFF * 100

   If Lightness = 0 Then
       If Darkness = 0 Then
           TintingAndShading = ""
       Else
           TintingAndShading = ", Darker " & Darkness & "%"
       End If
   Else
       TintingAndShading = ", Lighter " & Lightness & "%"
   End If
   
'ADDED CODE
   Dim lngr As Long
   Dim lngg As Long
   Dim lngb As Long
   Dim lngrgb As Long

   lngr = CLng("&H" & Mid$(ThemeColorHex, 7, 2))
   lngg = CLng("&H" & Mid$(ThemeColorHex, 5, 2))
   lngb = CLng("&H" & Mid$(ThemeColorHex, 3, 2))
   lngrgb = RGB(lngr, lngg, lngb)

   HSLtoRGB lngrgb, 0, 0, 0.25
'END ADDED CODE

   ' Display the colour instead of the scheme name
   MsgBox "The colour is: " & _
          "Red: " & CLng("&H" & Mid$(ThemeColorHex, 7, 2)) & _
          ", Green: " & CLng("&H" & Mid$(ThemeColorHex, 5, 2)) & _
          ", Blue: " & CLng("&H" & Mid$(ThemeColorHex, 3, 2)) & _
          TintingAndShading

End Sub

Sub HSLtoRGB(RGB As Long, H As Double, S As Double, L As Double)

   Dim R As Double
   Dim G As Double
   Dim B As Double

   Dim HR As Double
   Dim HG As Double
   Dim HB As Double

   Dim X As Double
   Dim Y As Double

   If S = 0 Then

       R = L
       G = L
       B = L

   Else

       Select Case L
           Case Is < 0.5: X = L * (1 + S)
           Case Else:     X = L + S - (L * S)
       End Select

       Y = 2 * L - X

       HR = IIf(H > 2 / 3, H - 2 / 3, H + 1 / 3)
       HG = H
       HB = IIf(H < 1 / 3, H + 2 / 3, H - 1 / 3)

       R = H2C(X, Y, HR)
       G = H2C(X, Y, HG)
       B = H2C(X, Y, HB)

   End If

   RGB = CLng("&H00" & _
              Right$("0" & Hex$(Round(B * 255)), 2) & _
              Right$("0" & Hex$(Round(G * 255)), 2) & _
              Right$("0" & Hex$(Round(R * 255)), 2))

'ADDED CODE
   MsgBox RGB
'returns 4210752

End Sub