Speed squares are great little tools. I use mine all the time and have been meaning to add this upgrade since I've owned it. Usually I use it to mark simple 90 deg and 45 deg angles. I use it almost as often for drawing a line parallel to a straight edge, and occasionally have been known to use it for finding an unknown angle, or marking non-standard angles. It is not the most precise tool for doing this, but lives up to its moniker... It handles the job quickly... when you can read it.
I've been blessed with good eyesight, but occasionally it's too chilly or windy to work in the garage with the door wide open... and out of consideration for the neighbors I often close the garage door when using my loudest tools, leaving me in a less-than-ideal lighting situation. I know this would be a helpful modification for my dad and others who are finding themselves staring longer and longer at even the high-contrast black-on-yellow tape measures to figure out which mark is 1/8" and which is 1/16".
Step 1: Tools and Materials
1. Spray paint in a contrasting color.
2. A couple of rags.
3. Mineral spirits.
Alternatively, you could use a water-based primer (adheres to plastic *better* than topcoat paint).
*see note in picture about adhesion
Then you could use a rag dampened with water and omit the mineral spirits.
Another alternative would be a 'ceramic marker,' 'grease pencil,' or even a crayon. (in middle school, we used to customize our locker locks with a crayon, by rubbing it over the indented numerals and hash-marks, then buffing off the extra on the dial face with a paper towel. The principle following is the same.
Step 2: Add Contrasting Color
Spray paints usually come with the instruction: "several light coats are better than a single heavy coat." Ignore those instructions for this purpose. If the coat is fairly thick and still wet, it's much easier to bring back the orange. If the spray paint dries before you get to the next step....
DON'T SAY I DIDN'T WARN YOU!!!
...you may have to use chemicals harsher than mineral spirits, or even sandpaper and end up making your speed square even less precise.
Step 3: What an Ugly Tool!
BEFORE THE PAINT DRIES, take one dry rag destined for the garbage anyway (or a paper towel) and wipe off as much of the paint as possible. This reduces the amount of mineral spirits needed in the next step as well as making the next step a little neater.
Step 4: Contrast Achieved!
QUICKLY (the longer the paint dries, the harder it is to remove) take a rag LIGHTLY DAMPENED with the mineral spirits (soaking-wet will likely wash the paint from the indented measurement markings and numbers that you've just spent the last 3 1/2 minutes applying) and wipe the remaining dark film from the flat surfaces of the square.