If the image sequence is correct, FFmpeg (or more often: render options) are at fault. To learn that, instruct the user to render as Image Sequence. You can put a white outline on your cursor by setting 'Thickmouseedges' to true. First thing to find out is whether the issue happens on Krita’s or FFmpeg’s side (Krita saves all the frames, then FFmpeg is used to render a video using this sequence of images).White is the one of the best colors for a cursor because it can be easily seen over any background in game, however, on Floating Islands, a light grey is usually desired.

Try different color strengths and see what you can create!

These are some of the basic colors that can be created. Here is a list of some basic colors that can be set and the values to create them before 1.3: