Why did WWII prop aircraft have colored prop tips?
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