Since Lunar and Planetary surface details and atmospheric phenomena reflect sunlight as contrasting colors, you can use color filters to accentuate subtle details for planets, comets, and the Moon by selectively increasing contrast between features and markings of different hues.
The higher light transmission of a 40% filter (compared to a 13% filter) makes it ideal for smaller scopes and planets.
By using various colour filters, you can make planetary detail easier to see, and also by stacking multiple filters to combine filtration effects, you can get unique results.
The filters screw on to the thread at the bottom of an eyepieces. They can also be used with filter wheels and/or filter sliders for quick and convenient swapping during viewing sessions to compare differently filtered views.