I think that this Tonga stamp was printed with fugitive ink. The fact that it's lilac supports this; lilac and green were the only two fugitive ink colours available in the Victorian period and set quite a precedent in stamps.
kopenhagen may well be correct in suggesting that part of the stamp was protected from sunlight in some way, but the pattern of the fading looks more like the effect of the stamp being soaked off its envelope. The lilac would have washed away in water if thoroughly soaked. If it was delicately soaked off the envelope to form part of a stamp collection, that's exactly the sort of fading away of the lilac that I'd expect to see. Note especially bottom left corner where the lilac is still visible (although not as vividly as all along the right side of the stamp), which is hard to explain in terms of fading in sunlight. Also see the boundary areas where the fading from lilac to blue occurs. They don't look sharp enough (to me) to be the result of something covering half of the stamp, leaving the other half to fade. it's more a wash of colour that is entirely consistent with fugitive ink fading from a little soaking.
