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Notice that in this mockup of Dolphin, the file browser area, the area where you can actually see and interact with files, is blue, like the filter bar. It is a lighter shade, but still the same color:



This is because there is currently a filter in place. The idea is that whenever you turn on a filter, the file browser gets a blue tinge. I know there is a common issue, one that has happened to me on numerous occasions, where you set a filter and then forget it there. You then cannot find files you know are present until you clear the filter. It is quite annoying. This color-coding is a way to avoid that confusion. You will always know there is a filter because the file browser is no longer white. And you know exactly what has changed because it is the same color as the filter bar itself.

Although there are no mockups of them, this same principle can be applied to other similar situations. The colors listed here are only suggestions, you could use any combination of colors you want. But I felt these colors are different enough that they shouldn't be easily mistaken (at least for people who aren't color-blind). So for instance if you are doing a search, or browsing a saved search, the file browser would turn green. If you are in a read-only folder, it would turn red. If you are in a network or web folder, it would turn gray.

If you have more than one of these applying to the same folder, I think there are two options. One is stripes, which I think would look awful. The other is to have the most important color apply to the file browser (I am not sure which are most important), and all the others have their colors applied as concentric borders lining the inner edge of the file browser area.



This is part of my series of Dolphin ideas

Yes, this idea is very good. I'd really like to have such filter option.

This might cause problems with dark themes. Unless there would be added a new color setting in kde color theme, and using a hard coded one it might end up in
no filter - dark background, light text.
filter - light background, light text = zero readability.

A better way would be to add inner glow files containment about 5px size, using users preferred highlight color.

Maybe invert text and background colors, this way theme colors (dark/light or blue/green)used are irrelevant

maybe inversion of theme colors when a filter is enabled