Why do propeller aircraft from World War II and similar planes sometimes have the tips of the props colored? I see it on single engine planes like the P-51 as well as bigger, multi-engine planes such as the B-17.
That is not unique to WW2 planes, this is fairly common on most prop planes. It is to increase the visibility of a running (spinning) propeller to help people on the ground from accidentally coming in contact with them. The unpainted propellers on the large aircraft may be more related to wartime haste than anything else.
7. Why are propellers painted?
The faces of the propeller blades toward the pilot are painted non-reflective black so the spinning propeller is not seen as a shiny, hypnotic disc. The tips on the opposite side are painted bright colors so the spinning propeller can be more easily seen to warn those on the ground about walking into it.
The prop tips are painted for visibility, to show that the prop is turning. This is especially important when the aircraft engine will be running in close proximity to people on the ground, but is also helpful in alerting the crew of other aircraft that the engine is running, and the aircraft might be moving soon.
For visibility, the tip color should be in high contrast to the blade, typically white on black.
Some operators paint the prop tips in an alternating pattern for a strobe effect for heightened visibility. This is especially helpful when operating in areas where people on the ground may be unused to being around aircraft; the strobe effect is an attention-getter. A three bladed prop painted in this way might use a pattern of one, two, and three stripes, or two, three, and four stripes.
Here is an example of what this pattern would look like on a 206:
Source: own work
This is what the pattern on a Quest Kodiak looks like in motion:
Source: excerpt from original photo by Dave Forney
Visibility. Even on modern aircraft. On the flight decks of carriers for example. The E-2C and C-2A's prop tips are painted with a unique paint. As the prop turns, the friction between the prop and the air cause the paint to glow making it easier to see.
The aerospace and automotive-grade paint is applied to the tips of aircraft propeller blades, helicopter main and tail rotors, and other rotating objects to allow personnel to avoid injury or death from coming into contact with them in low-light situations.
The paint was developed under a Naval Air Systems Command (NAVAIR) contract as part of the Navy’s Small Business Innovation Research (SBIR) program.
Source—The [above] un-retouched photos show how a propeller with its blade tips painted with DHi’s PL safety paint (seen as two green rings) compares to a prop painted with traditional red and white safety marking paint (which is nearly invisible).
As mentioned above, the bright paint is there to make it easier for ground personnel to spot the edges of the propeller arc in daylight operations. The picture of the early B-17s looks late pre-war, when that was the style for the Air Corps. Later example from the war do add this hi-viz paint to the tips of props. I guess there were enough accidents associated with propellers that it became imperative to paint props this way for safety reasons.
It's not limited to WW II era airplanes. Just about all propellers used today use this hi-viz paint on the tips, like the current production sample of an SR22.