Does a highly visible frame colour improve safety significantly?

It wouldn’t make much difference from the rear. All a driver could see from the rear is the top of your seat stays, which aren’t very large. Same goes for the front. From the side, it would make more of a difference, but I wouldn’t say it’s significant enough to justify the fancy paint job.

Of course, if you like the way the fancy paint looks as well, that might make it justifiable.

Studies have shown that wearing hi-viz colours, particularly on moving extremities such as one's feet

I have a better idea. Let's move the hi-viz colours from the feet to the pedals, so that you don't need to choose your clothes based on having hi-viz colours. Let's also make the hi-viz colours retroreflective, i.e. all of the light shining on them is reflected to the exactly same direction, no matter where the light is coming from.

What you get is pedal reflectors.

Flashing lights, also dramatically increase the chances of being seen by the driver of an approaching vehicle.

Let's make a thought experiment.

What if every car on the road would have a flashing headlight?

I think you'll realize that the idea is not good.

Does having a bicycle frame in a bright colour also dramatically reduce the probability of being hit by a vehicle?

A typical bicycle frame main tube is perhaps 30mm diameter and 55cm length. There are three such tubes. Their total area is 0.05 square meters per side thus.

A typical bicycle chainstay/seatstay tube is perhaps 10mm diameter and 50cm length. There are four such tubes. Their total area is 0.02 square meters per side thus.

You get 0.07 square meters of surface per side.

Now, a problem is that a car approaching a bicycle is commonly seeing the bicycle at a small angle, let's say 5 degrees. This means the effective area is not 0.07 square meters but rather 0.07 * sin(5*pi/180) square meters or 0.006 square meters.

A frame will reflect perhaps 5% of the light back to the direction where it came from. The rest is scattered to other directions. So if this is the case, a frame corresponds to a 3 square centimeter retroreflector.

One of my rear retroreflectors is 4 cm x 4 cm, i.e. 16 square centimeters. I have two such retroreflectors for a 32 square centimeter total area.

The two retroreflectors alone are therefore 10x more efficient than frame color.

My cycling jacket is about 40 cm x 50 cm when seen from the rear. It is seen directly from behind, so a small angle does not decrease its effective size. It is 2000 square centimetres. If it reflects 5% of the light back to where it came from, it is equal to 100 square centimeter retroreflector. This is 3x more than the retroreflectors I have on my bike.

Your idea is worthless. Bicycle frame tubes are so small and seen at such an angle that they don't contribute to visibility.

What contributes are, from most important to least important:

• Lights
• Cycling jacket being of bright color
• Retroreflectors

I found a nice product that is a roll of shiny black tape that reflects very brightly at night and would just look like a faint black stripe on your black frame when not reflecting. It works during the day too. It can be applied to any part of the frame, cut to shape, applied to your helmet, etc.

That being said, being visible is dependent on so many factors beyond how much flashing and reflecting you and your equipment do. Drivers have to be aware of many obstacles to avoid, and doing so requires training of the mind. There are low curbs that arent very reflective that can damage an automobile if hit, garbage containers on the shoulder, mail boxes, trees and bushes, and so much other stuff. So putting it in perspective, I put little confidence in how visible I think I am to drivers, and try to be as vigilant as possible, ride as smartly as possible and try to not be in a position to get hit when ever possible. I might be dressed like Las Vegas Boulevard, but a driver staring down at his/her phone isn't going to know I was there until it's too late. How well trained are drivers to be aware of bicyclists and really pay attention adequately while driving to all safety factors?