Red appears on more national flags than any other color, and for reasons that split fairly cleanly into a few traditions. Revolutionary movements reached for it as the color of blood spilled for independence. Communist states adopted it as a direct ideological symbol. Colonial powers like Britain and Spain carried it into the flags of their former territories. And a number of countries settled on red simply because it’s bold, visible at a distance, and easy to reproduce.
Revolution and independence
France · United States · Mexico · Turkey · Indonesia · Peru · Chile · Poland · Austria · Ireland
Communist and socialist heritage
China · Vietnam · North Korea · Laos · Cuba · Angola · Mozambique
Pan-African colors
Ghana · Senegal · Mali · Guinea · Cameroon · Benin · Ethiopia · Togo · Burkina Faso · Zimbabwe · Kenya · Malawi · South Sudan · Sudan · Uganda · Rwanda (Rwanda’s newer flag actually dropped red deliberately, worth double-checking before including)
Pan-Arab colors
Egypt · Iraq · Syria · Yemen · Jordan · Kuwait · United Arab Emirates · [Palestine, if added later]
Colonial and naval heritage (British and Spanish red)
United Kingdom · Canada · Switzerland · Denmark · Spain · Portugal · Nepal
Everywhere else red shows up prominently
Japan · South Korea · Bangladesh · Sri Lanka · Singapore · Malaysia · Philippines · Georgia · Latvia · Bahrain · Qatar · Monaco · Trinidad and Tobago · Panama