6. A corridor is 2 m wide and 8 m long. It is to be paved with 6 different colors of 1 m x 1 m whole tiles (each tile is a single color, and there are enough tiles of each color), with the requirement that adjacent tiles must be of different colors. The total number of different color schemes is ( ).
(A) $30^{8}$
(B) $30 \times 25^{7}$
(C) $30 \times 20^{7}$
(D) $30 \times 21^{7}$
6. D.

First, lay the first column (two tiles) with 30 ways. Then lay the second column, assuming the two cells in the first column are covered with colors $A$ and $B$ (as shown in Figure 1). The upper cell of the second column cannot be covered with color $A$. If it is covered with color $B$, there are 6-1 ways to lay it; if it is not covered with color $B$, there are $(6-2)^{2}$ ways to lay it. Therefore, there are 21 ways to lay the second column.

Similarly, if the previous column is laid, the next column will have 21 ways to lay it. Therefore, there are a total of $30 \times 21^{7}$ ways to lay the tiles.