Formatting Cell Based On Match In A Range

I have two sheets (Results and August) with numbers in column A, I want to change the color of the cell on Results if its contents matches the number on August. I was feeling adventurous today, so I tried writing some VBA code, but it changes the color of ALL the cells. What am I missing?

Sub FindMatches()
'Compares student numbers on Results sheet to those on August sheet; if match is found then highlights the student number
Dim Sht1Rng As Range
Dim Sht2Rng As Range
Set Sht1Rng = Worksheets("Results").Range("A1", Worksheets("Results").Range("A65536").End(xlUp))
Set Sht2Rng = Worksheets("August").Range("A1", Worksheets("August").Range("A65536").End(xlUp))
Set d = Nothing
For Each C In Sht1Rng
Set d = Sht2Rng. Find(C.Value, LookIn:=xlValues)
If Not d Is Nothing Then
Sht1Rng.Interior.ColorIndex = 10
Set d = Nothing
End If
Next C
End Sub

I suspect that I shouldn't be using sht1rng.Interior.Colorindex, as I think that may color the entire range? What should I use instead?