The origin of yellow being the color of liberalism

[Statilius the Epicurean]

These days though, yellow is associated with CLASSICAL liberalism. Social liberalism is blue in the US, red elsewhere. For example, the UK uses red to associate with the left wing Labour Party.

In the UK self-identifying 'social liberals' will be found mostly in the Liberal Democrats, who use the colour yellow or orange. Very few of them in Labour (e.g. Tony Blair identified as a socialist or social democrat).