It is known that any 7-colorable knot in 3-space is presented by a diagram assigned by four of the seven colors. In this paper, we prove the existence of a 7-colorable 2-knot in 4-space such that any non-trivial 7-coloring requires at least six of the seven colors.
"7-colored 2-knot diagram with six colors." Hiroshima Math. J. 44 (1) 63 - 74, March 2014. https://doi.org/10.32917/hmj/1395061557