My preferred approach is to apply block colours as neatly as possible to the various parts of the miniature (i.e. skin, clothing, armour, weapons), then wash the figure using a very dark brown (or mix of brown and black-grey). On most plastics, this provides rather good definition. I then return to pick out the most striking parts of the figure in the original tone and a slightly lighter shade.
So, for example, on a Roman legionary, I would work as follows:
After priming, paint the whole figure a bone white.
Cover skin areas in a bronzed or sun-burnt skintone.
Paint belts and sandals leather brown.
Paint pila and shield reverse a darker brown.
Paint shield front a striking colour to ease unit identification.
Paint in metallics.
Wash the entire figure using a dark brown mix (I usually take Vallejo Sepia Ink thinned down in a ratio of 1:5 or 1:6 parts ink/water).
After that has dried, I pick out the most pronounced skin areas (lower arms, shins and toes) with a very fine brush and sunny skintone. Also, the largest metal areas (swords, pila tips, helmets) might benefit from a metallic touchup.
Finally, I'll varnish using gloss varnish for an initial coat and a matt coat thereafter. Unfortunately, I don't have any ancients painted this way to show, but have painted numerous 15mm figures using this method. The method is time-saving and can be elaborated upon by adding highlights after the wash has been applied.
Generally, I find 20mm plastics to have finer definition (if done well) than 28s, but lacking the depth you find on the larger ones, especially regarding facial features. Hence my recommendation to go for a "unit approach" and highlight the most prominent areas only, especially limbs and large areas of single colours (tunics, shields etc.).