1. Black overruns the light squares
2. Pawn sacrifice to control the light squares
The same as dark square strategy, only applying to the light squares.
In position 1, white is missing his light square bishop, so it makes sense for black to conquer as many light squares on the queenside as possible with 15... a4. At some point in the future black also hopes to play ...f4, releasing his unopposed light square bishop on e6 to claim even more light squares.
In position 2 (Krush - N. Pert, 2001), black makes a bid for light square control by sacrificing a pawn with 11... b5!. After 12. cb, black will not only have the a8-e4 diagonal for his pieces, but also the b6 and perhaps even the d5 squares for his b8 knight.
See: 'Colour complex' and 'Dark square strategy'.