@Sebastian:
Most of the time you indeed can see the In and Out charts on the same graph without using the third 'both' colour.
But in the situation when the In and Out values are identical [or very close], the only way to have unambiguous graph is to use the 'both' colour. If you use either In or Out colour for such overlap (without 'both'), it would be unclear whether the two values are identical or the second value is zero.
If you want to have a strictly 2-colour graph and never have the situation when your Download and Upload values are similar, you can simply set 'data both' the same colour as your smaller traffic stream (usually that's Upload).