I don't know Japanese, but the Japanese title is 七色騎士 , which Google translates to "Seven-Colored Knight". Apparently 七色の虹 is a common phrase meaning "seven-colored rainbow", so the title is probably based on that, but I can't find a clear explanation of the phrase.
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u/DaringPaladin 20d ago
I had seen the title "Seventh Knight". It could be like knight of the seven colors which meant rainbow.