Video from PNGs with color background

ffmpeg -f lavfi -i color=c=gray:s=1920x1080:r=30 -r 30 -f image2 -s 1920x1080 -start_number 1 -i %04d.png -filter_complex "[0:v][1:v]overlay=shortest=1,format=yuv420p[out]" -map "[out]" -vcodec libx264 -crf 25 -pix_fmt yuv420p out.mp4