Filter to add 3dB per octave?
take any of the pinking filters you can find and swap the zeros with the poles.

e.g. here's my pinking filter with 3 real poles and 3 real zeros (all inside the unit circle):

$$ H(z) = A \frac{(z-q_1)(z-q_2)(z-q_3)}{(z-p_1)(z-p_2)(z-p_3)} $$

$p_1$ = 0.99572754,     $q_1$ = 0.98443604 

$p_2$ = 0.94790649,     $q_2$ = 0.83392334 

$p_3$ = 0.53567505,     $q_3$ = 0.07568359

just swap the poles with the zeros.