Early Quenya
iluqinga
noun. rainbow
kiris iluqingatelda
place name. Gully of the Rainbow Roof
ukku
noun. rainbow
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
In an early name list, Qenya cognate of G. Cris a Teld Quing Ilon, precursor to S. Cirith Ninniach (PE13/101). It is a combination of kiris “cleft”, iluqinga “rainbow” and telda “(having a) roof”, as suggested by Christopher Tolkien (LT2A/Teld Quing Ilon). In one early name list it appeared beside a variant form Kiris Ilqintelimba (PE13/101) where the second element contained telimbo “canopy”. Yet another variant form, Ilwerantelimba appeared in a different name list (PE15/21), with the initial element ilweran(ta) “rainbow”.