"Rainbow" was the first written review for Hamasaki by American journalist Adam Greenberg for Allmusic.Awarding it three-and-a-half stars out of five, he commented that “"Rainbow" came after a string of high-charting albums, itself not reaching the same heights thanks to some interesting but uneven experimentation with her sound.” Although he criticized the production and certain fillers, he concluded “The album doesn't have as much refinement as much of Hamasaki's later albums, but it shows the progression of her skills and vocal abilities.” He listed "Rainbow" as one of Hamasaki's best albums, alongside "Secret" (2005) and "" (2008).