Red is a very negative colour and normally signals danger in the western world.
So I would think when we see red for shares losing value, we might feel more likely to have impulsive emotional reaction.
As such it might be better to use shades of red, with a big drop, e.g. 5% showing a bright danger red. While a smaller movement less than 5% showings a darker red that is almost black.
Would you want it the other way, so different shades of green when it’s positive in case people think 0.01% increase is amazingly good?
Neither +/- 0.1% is actionable, so what is it’s purpose?
Good point. How about amber between +/- 1% say, as a stock flipping to green or red on a 0.01% change is a little absurd. Stocks that are green at present are not even in profit anyway because tax and charges are not included.
I always liked Warren Buffet’s rather simplistic maxim: Never lose money
For retail investors never sell when stocks are red seems like a good rule to live by (if you buy solid companies). Once Freetrade are bigger and have the data, I’d be really interested to see an anonymised analysis from free trade of reasons investors lose - I imagine doing too much would come top of the list.
After Friday I’d be all for a giant 
covering the whole screen so I can’t see how bad things are 
I’m about 2% down already, and I’m only tracking the markets. I don’t care what shade of green we use, any green at the moment would be lovely.