White is the most common secondary color in flag design worldwide, even though almost no national flag is white on its own. Its meaning stays fairly consistent across cultures too, more so than most other colors on this list: peace, purity, and honesty show up again and again as the stated reasoning, whether the flag in question is a European tricolor, a Pan-Arab design, or a Nordic cross.
European tricolor tradition
France · Italy · Ireland · Netherlands · Luxembourg · Hungary · Bulgaria · Croatia · Slovenia · Slovakia · Serbia · Russia · Estonia
Nordic cross flags
Denmark · Sweden · Norway · Finland · Iceland
Pan-Arab colors, peace and purity
Egypt · Syria · Yemen · Jordan · Kuwait · United Arab Emirates · Sudan · Iraq
Pan-African colors, peace and unity
Ghana · Ethiopia · Rwanda · South Sudan · Guinea-Bissau · Central African Republic · Sierra Leone
East Asia
Japan · South Korea · Laos · Bangladesh
Everywhere else white shows up prominently
Canada · Spain · Portugal · Poland · Austria · Peru · Argentina · Uruguay · Chile · Panama · Nepal · Israel · Singapore · Indonesia · Monaco · Thailand