When completely re-doing the lettering e.g. on de-bossed rulers or dials, a simple ivory or off-white paint (or wax) works fine. On quite a lot of old calculators not all, but only some of the digits will be gone or damaged. To limit change to the item only the missing bits can be re-done.
The originally white lettering will however have discoloured with age - from natural yellowing and decades of dirt embedded into the surface. A good, effective way to re-fill these missing digits is to mix a colour-matched latex paint. White (or off-white) latex wall-paint as sold in testers is a good source of paint in small quantities.
In a small mixing-tray, some of the white latex paint can be mixed with a water-based transparent ink to match the remaining original. Painting a small scrap of paper to hold next to the remaining lettering for checking colour can be helpful, before adding more yellow, sienna, black, etc. A decent match can usually be achieved. (Because different colorants act different under different lighting, it may be good to do this in daylight.)
It is often surprising how yellow the 'white' digits really are.
Using a small brush ('camel-hair brush'), the recessed numbers can be filled with latex paint. Some digits that are particularly dirty can also be gone over to lighten them a little. At this stage, do not worry about paint getting on to surrounding area - simply placing a large blob of paint on a digit would be fine even. Overpainting is not a huge problem, it just makes for more cleaning to be done afterwards.
After giving the paint a minute or so to dry, the excess can be wiped off with a damp cloth. First lightly wetting the surface of the digit (wet finger) and then wiping with the cloth also works fine. Keep the cloth taut and flat, to not scoop paint out of the filled digits. The paint in the recessed writing should remain, latex-paint on the surrounding surface will wipe off without leaving a trace. In case of pitted or scratched surrounding, excess paint can be removed with a toothpick.
The variation in yellowing between the original digits is usually larger than the difference with the newly painted lettering. When the mask is again fitted over the numbering wheels, any small colour differences become even less noticeable.
Results are again readable, and the appearance is still in-character for a decades-old machine :-)