The median blur brush as well as filter and live filter layer can be used to create Paint effects very quickly with colorful images in Affinity Photo. Select a colorful image and then go to the tools panel and select the tool, apply the tool to a localized area.
You can use different brush widths as well as different types of brushes such as oils, charcoal etc. You can also use the filter and that means the result is applied over the entire image.
There is only one setting, the radius (it can be used interactive as well as via the slider and also via the edit field). Set the value to a fairly low setting and the result is a very water color paint like result with the colors all smudge by water and streaks. Set the value of the filter too high and the entire image will be blurred. It is useful for create flattened colors as well as blurring out color noise.
You can also use it with live filter layers resulting in changes made to a selected layer or to all layers. You can also use that with blending modes and opacity so you may find it is even more amazing when set in linear burn than using it in normal blending. As it is live, you can always double click the entry in the layers panel to modify it at any point. You can also add multiple layers with the same live filter layer and blend those together as well.
You can also create many interesting colorful designs by using the channels panel found in the view menu and studio. You can apply the filter or brush stroke in the red channel and then change the settings and apply in the green or blue etc. You can then go back to RGB and continue to modify the image
You can also select a particular part of the image and use the brush / filter there. You can use the rectangular marquee tool as well as the select menu and tonal range and mid tones say and apply it to that or perhaps invert that selection and apply a different setting to that area. You can use the filter to modify the selection itself by going to the select menu and edit selection as a layer. Apply the filter to that instead. Once you have exited out of editing the selection apply other filters etc to the generated selection.