The below code will change all the line colours in slide 2 to red in Powerpoint 2007:
Public Sub ChangeLineColours()
Dim shp As Shape
For Each shp In ActivePresentation.Slides(2).Shapes '<~~ Change '2' to whichever slide you want to loop through
If shp.Type = msoLine Then
shp.Line.ForeColor.RGB = RGB(255, 0, 0)
End If
Next shp
End Sub