Yet again, Americans choose to be the odd ones 🤔

Canada's convention is pretty much universal. Even the Communist countries used, and still use, red but in democratic countries with more than one Party, it's generally right-wing blue, left-wing red.

The UK also has its Liberal-Democrat Party, left-of-centre, originally formed by members of the former Liberal Party and disaffected Labour Party politicians. Its campaigning colour is orange.

UKIP was yellow and purple. The Green Party is, of course, green at least on its campaign leaflets. I don't know what the others, the SNP, Plaid Cymry, etc., use.

...

You mention the Metric System, or to be pedantic now the "Systèmè Internationale" based on metric measures and agreed by the International Standards Organisation, of which the USA is one of its very many full members. American scientists and many engineers now use the SI units, and any manufactured goods imported by America, such as cars, domestic appliances and industrial machinery, will have been made to metric dimensions unless to some special - and probably cost-adding - order.

Britain has a few exemptions but otherwise is all metric. The roads and railways still use the Statute Mile and Yards (and Chains, on the railways) for distances, MPH for speeds; and although vehicle fuels are sold in litres their fuel consumption is still quoted in miles-per-[UK] gallon, and power in Horsepower. Pubs are allowed to dispense draught ales and cider by the pint and half-pint, but spirits must be dispensed in millilitre units. All other foods and drinks are sold metrically even if the numbers are actually gramme equivalents of Avoirdupois masses.

German car manufacturers still quote car powers in something called the "PS", which is metric but obsolescent. The ISO unit of power is of course the Watt, and its threes-powers of ten, multiples and divisions. I don't understand why car makers use the litre rather than cubic metre to describe load space: it's a car not an aquarium!

The UK and US Gallon differ because the latter preserves an ancient vintner's measure long since obsolete everywhere else. I wonder if this reflects the early settlers using wine casks to store food and drinking-water on their ships for the long trans-Atlantic voyage.