Photoshop :: Successively Smaller Color Indexing For Painting

How do i reduce the color index of a vibrant painting in successive stages (specifically from 100->50->25->16->8) in order to (hopefully) maintain good quality as its palette limits down, instead of just subjecting the orig to 'Color Index: 8' and get horrible results. What are common methods in doing it:  a) with the entire image at once  b) using specific selections of particular color areas at a time (a little section of oranges here, a little area of greens there) and then 'mixing' the results with a reduction done to the whole image   and c) common methods, tricks, etc. working in 'reverse' by reducing an image's color manually using filters, br/contr., hue/sat. toggles, etc.    I only have Ps7 and am not too skilled in using layer blending modes, gradients, paths. I get satisfactory results so far just using mostly Normal layer blend and adjustment layers using custom-mask selections.