Stop generating 8-bit JPEGs automatically?

[rrt]

I just spent a while trying to work out why a film I constructed from
JPEGs using ffmpeg was in B&W.

I finally discovered that it was because the first frame was an 8-bit
(greyscale) JPEG, generated by

convert -size 1024x768 xc:black black.jpg

I worked around the problem by generating a PNG, but it's rather
annoying that imagemagick produces this little-used format just
because I said "black" rather than "green".

There's a mailing list message that suggests that the developers might
be amenable to fixing this:

http://www.imagemagick.org/pipermail/ma ... 01536.html

Would it be possible to change this? Or at least make it possible to force a 24-bit JPEG to be output? According to the message I quote, this is (or at least was not in 2002) possible; and if I add "-depth 24" to the command-line above it certainly doesn't help.

[fmw42]

I don't know if this is relevant, but from http://www.imagemagick.org/Usage/formats/#jpg_write

it suggests that adding -type truetype may override the 8-bit grayscale and force 24-bits