This is the basic principle, I assume that customizing this code is not what you are asking (as no details about this were provided):
Sub Colors()
With Range("A1")
.Value = "Test"
.Characters(2, 2).Font.Color = vbGreen
End With
End Sub
Small description although it speaks quite for itself: the first “2” refers to the first character that needs to be colored, the second “2” refers to the length.