This article is from the Acoustics FAQ, by Andrew Silverman with numerous contributions by others.
The power spectral density of white noise is independent of frequency. Since there is essentially the same energy between any two identical frequency intervals (for example 84-86Hz and 543-545Hz), white noise narrow band FFT analysis will show as flat. However octave band analysis will show the level to rise by 3dB per octave because each band has twice the frequency range of the preceding octave.
Pink noise is often produced by filtering white noise and has the same power within each octave. Narrow band analysis will show a fall in level with increasing frequency, but third-octave band or octave band analysis will be flat.
see Joseph S. Wisniewski's Colors of noise FAQ at:- http://capella.dur.ac.uk/doug/noisecols13.txt