FFmpeg change overlay color (with audiovisualiser)
Use

ffmpeg -y -i music.mp3 -loop 1 -i image -filter_complex "[0:a]showwaves=s=1280x175:colors=Red:mode=p2p,format=yuva420p,split=2[w][b];[b]format=yuv420p,drawbox=c=blue:t=fill[b];[b][w]overlay[wv];[1:v][wv]overlay=0:200:shortest=1[outv]" -map "[outv]" -pix_fmt yuv420p -map 0:a -c:v libx264 -c:a copy output_bg.mp4

The showwaves will make the background transparent if the filter can output to a frame format with alpha. yuv420p does not have alpha, so the value of the format filter should be changed to a format that does, like yuva420p. Then this picture is cloned; one copy is made opaque and has a color drawn on it. The other copy with the transparent BG is overlaid on top. Finally, this result is overlaid on the image.

It's better to limit the video stream within the overlay filter, if possible. So, -shortest removed.