a(1) = 5, since in the 5cell, each cell has a common face with every other cell (analogous to the tetrahedron, where each face has a common edge with every other face).
a(2) = 4, since in the 8cell, each cell has a common face with every other cell except its "opposite" cell (analogous to the cube, where each face has a common edge with every other face except its opposite face).
a(3) = 2, since the 16cell's dual graph has no oddedge cycles (analogous to the octahedron's dual graph having no oddedge cycles).
a(4) = 3, since the 24cell has at least one 3color solution, and its dual graph has a 3vertex subgraph with no 2color solution.
a(5) = 5, since the 120cell has at least one 5color solution, and its dual graph has a 30vertex subgraph with no 4color solution.
a(6) = 3, since the 600cell has at least one 3color solution, and its dual graph has a 5vertex subgraph with no 2color solution.
