David Carroll gave an admirable (but extended and hard to snip) description of
hot-stamping with foil, and suggested adapting it for marking the ruler: "You
might try heating your punches and using Carbon paper to set the color?"
Short version: Carbon paper used cold with impact seems to give very good
results. I deprecate usefullness of hot stamping for this project.

Long version: Hot stamping is basic edition bookbinding technology so I've got
some experience with it. Gold and color foils for hot-stamping were developed in
(I believe) the 1920s or 1930s. You can get them readily, if you want them, from
TALAS, in a much narrower range of colors than are made but in relatively small
quantity:

www.talasonline.com
and search for "hot stamping foils" on the site.

The difficulty is the temperature needed, which is around 200 degrees
Fahrenheit. Too hot to hold in the hand, too difficult to control a normal punch
with pliers or suchlike, and it has to be just right. Foil is made to be used
with machines with controlled temperature and jigs for placement, since with the
foil in place on the substrate you can't see where you are going to stamp. You
can use foil with hand tools, but it takes practice.  Binders' hand tools are
set in wooden handles, and you heat them on a hotplate or alcohol lamp. You
deliberately overheat them a bit, then cool on a wet hunk of cotton until the
sizzle sounds right, just the way clothing irons were heated and judged by
spitting on them back in the day. In striking the tools your variables are
temperature, pressure, and dwell (length of time on the surface). If one
variable changes, the others will change too. And the optimum combination will
vary with the humidity and temperature of the room. In general with foil a
short, hard strike with a high temperature will work best-- but not too hot or
too hard or too short. You could make wood handles for your punches, or even one
handle that would take different punches, but you still have the problem of not
being able to see the placement on the substrate. I'd say it is a pretty steep
learning curve. Maybe not the steep side of Half-Dome, but steep.

Carbon paper used to be a standby back-up for fine binders working on leather.
Most veg-tanned leathers will darken attractively if tooled wet and slightly
hot. It's tricky, though, and when the impression didn't darken evenly or
adequately, one technique was to touch a hot tool to a piece of carbon paper,
picking up the black color on the face, and re-tool with that. This was less
than ideal because the wax carrier of the carbon paper stayed permanently
sticky, and accumulated dust over time; if the binding was to be varnished (done
in the 19th century, not today, basically very thin shellac) the varnish would
seal the wax and pigment in. Despite the dust-magnet problem, he technique
basically worked very well. However, forty or fifty years ago they came out with
"smudgeless" carbon papers, and hot tools don't pick up color from the new kind
(or if they do, it is just little partial blobs). My binding teacher has a
treasured single box of smudgy carbon paper from the old days, but I have none,
though I have once or twice begged a scrap from her for an important project.
You would never put carbon paper face-down on a binding and tool through it,
because the proper placement of the tool couldn't be found, and because the
smudgy carbon would irreversibly soil the surface of the binding.
Modern smudgeless carbon paper does seem to me to have possibilities for the
current project, though. Remember that it is made for use in typewriters, which
transfer the color by impact with a steel punch. Here's a procedure to try:
stamp the wood without color to get the location. Put a piece of carbon paper
over the place, face down. Gently feel the punch back into its impression. Then
strike it again with the hammer, replicating the strike of the typewriter key.
The more I think about that one, the more it feels like a possibility.
In fact: it seemed a good enough theory that I just went and tried it on a scrap
of pine, and it worked pretty well. A hard blow didn't work; it seems that it is
more effective to make the initial strike gently for a shallow impression, and
then striking with carbon paper in place seems to transfer better with a light
blow than a heavy one. And I found that the results were better if I repeated
the strike with carbon paper several times, using fresh bits of the paper. Good
possibilities on this method, if repeated (gentle) triking doesn't damage the
thine wood of the rule.
Tom Conroy