A very non historical answer. For what it's worth. I'm answering the question "why do people like or notice red colour more?" and extrapolate to flags. Many of the assertions below are well recognised in evolutionary biology but I did not look for sources for each of them.
Most land plants are green because they use chloroplasts to power the synthesis of organic molecules from carbon dioxide (Calvin cycle). For land plants the applicable lineage of chloroplasts are green (red for some sea weeds chloroplasts).
Red is the complementary colour to green (as all artists well know). Which means that red and green offer the strongest contrast. Another pair of complementary colours is blue and yellow (=> Swedish flag anyone? :).
There is therefore an evolutionary pressure for plants to advertise ripen fruit as red and conversely hide immature fruit as still green. Fruit consumers (who pay for their consumption by disseminating the seeds) have synchronously progressively experienced the complementary evolutionary pressure to see from far away and to be attracted by red objects. For instance the animal species most receptive to red are African primates - while most other animal species don't see red at all. Notable exceptions are birds (heavy fruit and berry consumers) and crustaceans (not sure why because red is rare in the sea). Similarly aphids are attracted by light green (young poison free leaves) and pollinator insects by ultraviolet (to spot flowers).
So young primates like colours (ever noticed marketing techniques to have your kid grab their yoghurt pack in the shops?) because in their original rain forest habitat it's a competitive advantage to be attracted by vitamin and nutrient loaded fruit. We has human (with interestingly enough regional differences) like red.
It remains to be seen of course whether bees national flags predominantly contain ultraviolet motifs and aphids flags green ones. :)
About regional differences - this is my own interpretation here. Northern hemisphere (colder countries) flag have also a lot of blue because nice weather (blue sky) allows to synthesize more vitamin D and it's a competitive advantage for its inhabitants (originally coming from the African plains) to have blue eyes, light skin (with less melanin) and to research exposure to sun light.
Conversely African flags were light is plentiful have very little blue but are very colourful.