The radical for red and the kanji for red are the same thing!
Think of Fozzie Bear. He says "Wakka wakka wakka" a lot. Now, think of an evil, red (and I mean totally red) Fozzie Bear. He drops the "w" and just says "Aka Aka Aka," because he's totally evil.
Currently there are no similar looking kanji which exist in the WaniKani learning system