I agree with Rich Bliss, but it might be worth a try for raised sculpted items like buttons.
I have used acrylic paint pens for painting individual raised flagstones on terrain tiles; I tried the pens thinking the stiffer felt nib could help me be neater than with a brush. This worked out as I expected. Also, since the paint feeds from the pen's reservoir, there is no need to keep dipping to reload, like with a brush.
Note, I am not talking about the craft store / stationary store paint pens. I am talking about artist-grade paint pens. I already used Liquitex soft-body paints and so I selected Liquitex acrylic paint pens to have matching colors. Liquitex is a premium paint brand but I am willing to pay their prices to be sure of paint quality.
I also have experimented successfully with refilling the Liquitex pens when they run low and I've also used the fill-your-own empty paint pens (I forget the brands, but I've tried a couple). This is an example of the Liquitex product quality – I could use their media to thin their paint for use in pens and be sure there was no issue of clogging due to irregular pigment. (The paint formula I use was from a YouTube video and is 1:1 Liquitex Soft Body paint to Airbrush Media with about 5-10% Flow-Aid added.)
If you keep these pens sealed well and store them on their sides, they are less likely to dry out which is the main cause of flow problems during re-use. For the artist-grade pens, you can get replacement pen nibs, although I haven't done this yet.
Again this is always on terrain; I haven't used them on miniatures yet, although doubtless I will try it some day.
One learning issue I'll point out is the felt nib is less gentle than a brush; if you manipulate the pen wrong you can find yourself scraping with the nib and applying minimal paint flow. This can scrape off a layer of paint if it is only touch dry. (Oops!)