Centring Dutch leftist expressions of solidarity with two anti-colonial revolts – the 1926 Indonesian revolt and an attempted 1929 Curaçaoan revolution – this article examines how leftists in the European metropole sold solidarity with anti-colonialism across different political arenas and organizational levels. Relating their efforts specifically to the communist–social-democratic divisions that shaped various leftist political arenas in the period studied, we posit that an expression or practice of solidarity hinges not only upon agreements over ‘mutual interests’ among those directly involved in said relations of solidarity. As interwar left-wing groups depended on diverse coalitions and organized on different geographical scales, their claimed ‘mutual interests’ with anti-colonial causes had to be legitimized, or sold, across these scales. Using newspaper and archival sources, we first argue that solidarity represented a ‘flare signal’ through which colonial causes invited solidarity and support from abroad. Second, we argue that expressions and practices of anti-colonial solidarity, to Dutch interwar leftists, represented a tactic to forge alliances and discredit opponents on a scale of domestic politics, and to cultivate support and status in international networks. Dutch interwar leftist positions on anti-colonial solidarity, we propose, were profoundly shaped by the efforts of leftists from both the metropole and the colony to ‘sell’ claims of mutual interest across scales.