I am not talking about a glaze that gives a slight tint and then collects in the seams and cracks, but an overall color change with hints of the earlier paint.
This is very easily achieved using one or both of these techniques: wiping and abrasion.
The first is simply to selectively wipe at the paint as you apply it, with your brush or roller in one hand and a clean rag or piece of burlap in the other. With enamel or oil paint the cloth can be dampened in spirits or turpentine to increase the working window for this basic method from minutes to an hour or more (wear gloves and work in a well-ventilated place if doing it this way, especially if using turpentine).
The second is to paint, let the paint dry (see note 1 re. drying time) then abrade the paint from the piece where you want some previous colour to show through. Removing the paint can be done in part by scraping with various tools, but is most commonly done using sandpaper, steel wool and Scotch-Brite or an equivalent. Even stainless steel pot scourers can be used if the coarser scratches they leave are acceptable. See also note 2.
Of course there's no reason you have to use one or the other, both techniques can be used if desired.
Note 1, drying time: some guides call for waiting for the paint to fully dry, or cure, before starting abrading work, others suggest that you can start as soon as the paint is dry to the touch. Needless to say both work — the paint comes off either way — just slightly differently. And you can of course start as soon as the paint is touch-dry and then do further work later on when the paint has cured.
In practice I haven't found either to be the best approach and both have their place. With some paints they come off a little too easily when just dry, but this can be used deliberately for edges and around handles where you want to mimic the paint having worn through completely from long use. The effect is sometimes a bit coarse or crude, but this is easily made more subtle later on with finer abrasives and light rubbing.
Note 2, when working with waterbased paints: an additional aid for this technique that's sometimes advised is to rub a candle or piece of wax on edges, around knobs etc. where you want to show particular wear. The paint won't adhere to the wax and it is then much more easily abraded from the surface after being allowed to cure. This tip is particularly useful when using milk paint which is very tough and resilient when cured.
The usual advice with any techniques like this is to practice on a scrap piece first to get a feel for it. That's not really possible here, so if possible practice on the back or at least a less-visible portion of the piece before committing to the most visible areas.
