The names appear in my translation as: “de Rooie” (Red — with the note that “rooie” is colloquial Dutch for “rode”), “de Blauwe” (Blue”) for the boys, en “Witje” (White) en “Zwartje” (Black) for the girls. For the girls I went with nicknames without the article “de” but with the diminutive/endearment suffix “-je.” This ending is used to indicate that things are small, and can be used for males and females. ‘Jongen’ (young one) boy. ‘Jongetje’ small boy. But because Shiro and Kuro are girls, it seemed more natural to give them names ending in -je than names with ‘de’ (article ‘the’).
My translation of the passage quoted in Ika's post in Swedish and German is: “De twee jongens heetten Akamatsu en Ōmi — Rodeden en Blauwezee —, en de twee meisjes Shirane en Kurono — Wittewortel en Zwarteveld. Alleen de naam Tazaki — Veelpunt — had niets met kleuren te maken.”