A filter is not necessary but they are helpful.For example on a monochromatic color like gray or green,a filter which is a very thin mix of paint and spirits 90-10 helps bring about a subtle tone difference .On green you can use brown,tan,yellow to see what effect you like,for gray some use blue.On a German tri-color camo, the filter helps pull the three colors together,tans and browns work good for this.Filters are brushed on with a 2 hr wait between coats.you can mix your own or buy them pre mixed.Big suppliers are Mig Productions and AK Interactive.Look on their sites for instructions and tutorials on their products.