I need help I have applied washes before, but never filters. So, I have questions?

What is the mix for filters?

What colors do you use one a regular basis for filters?

When does the filter get applied?

Are fliter only for monocromatic paint or do they work with camo??

Thanks for your help!

I use light grey, for NATO camo. Tamiya buff, for earth colors. Monochromatic should be a mix of the base coat and a little lighter or darker shade added to it. You apply the filter, after the top coat and before the wash. I mix filters a 20:1(thinner:paint). There are several articles on this, including the painting and weathering sequence stickied at the top of this forum.

A filter is typically 95% thinner to 5% paint. A filter is just used to slightly alter the base color or give the base color a certain tint, so really any color could be used. However, I am guessing browns and greys are the most common colors used and I am sure other colors like greens and blues are used commonly as well. I think it just depends on the color of your base coat and the effect you want to achieve.
Yes, you can use filters for camo and it helps blend the colors or make the difference in colors less drastic.
I think many people apply filters after the basecoat and before the first wash (thats when I would apply it), but I am sure there are different times to apply them.
Here is a link to an article about filters by Mig Jimenez.
http://www.missing-lynx.com/rare_world/rw02.htm
Hope that helps. I am definetly not an expert and am pretty new to modeling myself.