Second Demension skin tones

I can't speak to Buford, but the idea was Phineas and Ferb in the Second Dimension were always hiding or hidden inside and didn't go out to play the way First Dimension Phineas and Ferb do, so their skin isn't as tanned. Perhaps Buford, who was resisting the resistance at the time, wasn't going outside as much, either. When they went to the later episode, the 2D boys were playing outside more, so their skin returned to the same tone as their 1D counterparts.