Born in Wales, Colwill made his debut for the national under-17 side in a 2–2 draw against Belarus on 10 October 2018.[6] He was selected for the Wales under-21 squad for a friendly match against Republic of Ireland on 25 March 2021 and made his debut for the side as a substitute during a 2–1 defeat.[7] On 30 May 2021, he was named in the Euro 2020 squad.[8] He made his senior national team debut on 2 June 2021 in a friendly against France, substituting Joe Morrell in the 83rd minute.[9]