Hi Daniel,
I'm not a specialist, but I'll try to explain.
Filters and washes are used a lot by Airplane and AFV's modelers and are part of the weathering process.
Basically, a filter is a controlled wash.
Generally, when we say "wash", then the most of the pigment will accumulate in deep recesses and only a small quantity will stay on the surface. This will give depth to the painted area. It is applied by flooding the area.
When we say "filter", then the most of the pigment stays on the surface, altering the color. We are controlling the wash by keeping the pigment on the surface, not allowing to run and accumulate in the deep recesses. We want a diluted mix, applied in multiple, thin, transparent layers. The effect is achieved gradually. So, the filter is a controlled wash.
The lapels are done with Andrea red paint set (ARPS). I painted the base color, shadows and highlights, as usually. I got harsh transitions between them and decided to apply a filter, to make those transitions smoother. I prepared a diluted mix, using the base color (ARPS No 1) and applied in small quantities and multiple, transparent layers. Each layer was applied on dry surface, bottom up.
This is an abbreviated explanation, but I hope it helps.
Let me know if you have questions.
Cheers,
Zeno