Matte repainting technique for action figures
First, if you haven't already, disassemble the figures. This will allow for easier reach of those parts that are usually obscured, a more even application of the paint, and a cleaner colouring around the joints.
You can carefully use a blow drier to make the plastic a little less rigid.
Wash the figure or parts with a little soap to get rid of any dirt that might prevent the paint from adhering to the surface.
This might also be a good time to trim or sand excessive plastic (like edges).
It's not always necessary, but I suggest priming the figures with an acrylic ground layer, preferably sprayed on (using an airbrush and NOT cans, as they contain solvents - an exception being Duplicolor Vinyl &amp; Fabric, as a user on the thefwoosh forums suggests, as this apparently does work on soft plastics) to retain a smooth surface, but at least applied in thin, diluted layers. 
Apart from allowing the paint to have a stronger bonding, this will also form a neutral 'canvas' to see how the colours work together, and to ascertain the colours won't differ a lot from how they are advertised (you can also try out the paints on similar material or parts that will be hard to see on the finished figure [like the soles of the feet]).





Now you can paint your action figure using matt(e) / flat acrylic paints.
You can find those in your local hobby store, or online. 


OR


Use regular acrylic paints, and seal them off with a matte finisher.
And again, if by hand, instead of using an airbrush, always paint in thin, diluted layers. Also make sure the sealer can be used for soft plastics/PVC, as some people experienced the layer getting tacky again after a while.
Noteworthy is that that same poster mentioned earlier suggests not using any sealer at all, as they always have a certain amount of sheen, even if advertised as matte or flat, with Testors Dullcote being the sole exception in that user's experience.


Always make sure you work in a dust-free environment.




Some additional resources to browse through for some advice:
https://www.reddit.com/r/ActionFigures/comments/5unmtp/best_paint_for_marvel_legends/
https://myfigurecollection.net/blog/7299
https://forum.rebelscum.com/threads/question-about-what-paints-to-use-to-custom-paint-figures.1089439/