俱舍論記 28.36

28.36

「皆無色故立無色名」者，釋第五句。空等四處皆無色故，立無色名。

As for "They are called 'formless' because they all lack form": this explains the fifth line. Because the four stages, such as [boundless] space, all lack form, they are given the name 'formless' (ārūpya).