For both the filters and the washes I used oil colors. These are more versatile than acrylics, because they allow more control on color shading and they have longer drying time, thus giving more time to fix any errors. On the other side, however, they can be used only with volatile solvents such as white spirit or turpentine.
I used van Dyck Brown oil color diluted with turpentine at 90% ratio. Before proceeding, you can coat the model with satin varnish, both to protect the underlying color and to improve the grip of the filter. After this, you apply the oil color with a large soft brush, removing the excesses. In this way you not only form subtle color variations, but also you uniform the color shades, softening the contrasts between the different hues.
For the washes, I diluted again the color with turpentine, this time to 50-60%. After that, I used a fine brush in order to put small drops of it into the recesses and around the protruding parts (just touch with the brush tip and the color will flow because of capillarity). With a brush soaked with clean turpentine I removed the excesses.
I also applied a wash on the wooden parts, in order to achieve a more realistic effect.
Note that the wash into the gun muzzle highlighted its rifling, previously invisible. This shows the level of detail of the brass barrel included in the kit (the actual diameter of the hole in the photo is about 1.6 mm, or 1/16"!).