Stroke (Both) adds color to both sides of the drawn line.
Prepare an illustration you would like to add borders to.
Open ① the Layer window, and select ② the current layer and drag over the layer that contains the image you would like to apply the filter to.
From ① the Tool Selection window, select ② Filter.
Select ① Stroke (Both).
Drag the ①Sliders to adjust the width of the borders.
Open ①Color Picker . Drag the ②Slider to adjust color.
When finished, return to the canvas with ①✓ button.
Finished.