Kept quiet since I'm far from a weathering guru.
but my experience is that on a smooth painted surface, Alcohol/India ink will tend to pool giving a mottled effect and will not be ideal
for darkening the color... It really works much better on a porous surface...
I think a better method would be to airbrush a thinned clear flat finish with a couple drops of black or darker grey paint, and slowly build up until you have the color you want.
As Tom said the Alcohol may damage the paint depending on the strength/duration of exposure, additionally if the item is dulcoated before hand the alcohol will cause the dullcote to whiten...