for %G in (*.png); do magick "%G" -transparent #FF00FF "batch\%G"; done

convert image -fuzz XX% -fill red -opaque white result

1. Extracting frames:

   mkdir anim && cd anim
   ffmpeg -ss 14:55 -i video.mkv -t 5 -s 480x270 -f image2 %03d.png

2. Selecting the frames (only 1 in 3, from frame 10 to frame 72): seq -f %03g.png 10 3 72
3. Creating the animation:

   convert -delay 1x8 `seq -f %03g.png 10 3 72` \
         -coalesce -layers OptimizeTransparency animation.gif

4. ImageMagick tricks:

   convert -fuzz 1% -delay 1x8 `seq -f %03g.png 10 3 72` \
         -coalesce -layers OptimizeTransparency animation.gif

   or even :

   convert -delay 1x8 `seq -f %03g.png 10 3 72` \
         -ordered-dither o8x8,8 \
         -coalesce -layers OptimizeTransparency \
         +map animation.gif