On the Underwood Champion typewriter, the carriage ruler is plain metal with a black scale. The lettering is very crisply recessed (etched?) and the ruler is also thicker than on most typewriters. Even so, the scale did lose some of the black coloring over its last 80-odd years.
Refreshing such scales is fortunately a fairly easy matter. Again using the advice in Teege's excellent book on typewriter maintenance, a proper wax crayon and a hair-dryer are all that is needed.
Using the hair-dryer to heat up the metal ruler to somewhere between 50 to 70 degrees Celcius (take care, do not burn fingers, use a rag or kitchen-towel to handle) and then quickly going over the scale with the wax. The wax will melt and fill the scale. (This etched, plain metal is ideal for re-filling markings. With painted metal a bit more care is needed to avoid heat-discolouring, even more care with plastic parts to avoid warping.)