Answer: $3^{n}+(-2)^{n+1}-1$
Let the $2 n$-gon have vertices $A_{1}, A_{2}, \ldots, A_{2 n}$, in that order. Consider the diagonals $d_{1}=\left(A_{1}, A_{n+1}\right)$, $d_{2}=\left(A_{2}, A_{n+2}\right), \cdots, d_{n}=\left(A_{n}, A_{2 n}\right)$. Suppose the three colors are red (R), green (G), and blue (B). Each diagonal can either be colored $(R, G),(G, R),(G, B),(B, G),(B, R)$, or $(R, B)$. We first choose one of the six colorings for $d_{1}$, which then constrains the possible colorings for $d_{2}$, which constrains the possible colorings for $d_{3}$, and so on. This graph shows the possible configurations; two pairs of colors are connected by an edge if they can be the colors for $d_{i}$ and $d_{i+1}$ for any $1 \leq i \leq n-1$.
Suppose without loss of generality that $d_{1}$ is colored $(R, G)$. (At the end, we multiply our answer by 6.) Then $d_{n}$ must be either $(R, G),(B, G)$, or $(R, B)$. Now, we simply need to count the number of paths of length $n-1$ within this graph from $(R, G)$ to one of these three points.
Suppose we are making a random walk of $n-1$ steps, where at each move we pick one of the three possible edges with probability $\frac{1}{3}$. We will calculate the probability that the walk ends at one of $(R, G)$, $(B, G)$, or $(R, B)$.
Let $a_{i}$ and $b_{i}$ be the probability that, after $i$ steps, we are at $(R, G)$ and $(B, G)$, respectively. By symmetry, $b_{i}$ is also the probability that we are at $(R, B)$ after $i$ steps.
Observe that after each move, the probability of arriving at either $(R, G)$ or $(G, R)$ will always be $\frac{1}{3}$. Therefore, the probability of being at $(G, R)$ after $i$ steps is $\frac{1}{3}-a_{i}$. Similarly, the probability of being at $(G, B)$ is $\frac{1}{3}-b_{i}$ and the probability of being at $(B, R)$ is $\frac{1}{3}-b_{i}$.
Now, for $i \geq 1$ we have the recurrences
$$
\begin{aligned}
a_{i+1} & =\frac{1}{3}\left(\left(\frac{1}{3}-a_{i}\right)+\left(\frac{1}{3}-b_{i}\right)+\left(\frac{1}{3}-b_{i}\right)\right) \\
& =\frac{1}{3}-\frac{1}{3} a_{i}-\frac{2}{3} b_{i} \\
b_{i+1} & =\frac{1}{3}\left(\left(\frac{1}{3}-a_{i}\right)+\left(\frac{1}{3}-b_{i}\right)+b_{i}\right) \\
& =\frac{2}{9}-\frac{1}{3} a_{i}
\end{aligned}
$$

So then
$$
\begin{aligned}
a_{i+2} & =\frac{1}{3}-\frac{1}{3} a_{i+1}-\frac{2}{3} b_{i+1} \\
a_{i+2} & =\frac{1}{3}-\frac{1}{3} a_{i+1}-\frac{2}{3}\left(\frac{2}{9}-\frac{1}{3} a_{i}\right) \\
a_{i+2} & =\frac{5}{27}-\frac{1}{3} a_{i+1}+\frac{2}{9} a_{i} \\
\left(a_{i+2}-\frac{1}{6}\right) & =-\frac{1}{3}\left(a_{i+1}-\frac{1}{6}\right)+\frac{2}{9}\left(a_{i}-\frac{1}{6}\right)
\end{aligned}
$$

This recurrence has a characteristic polynomial $x^{2}+\frac{1}{3} x-\frac{2}{9}$, which has roots $\frac{1}{3}$ and $-\frac{2}{3}$. We can write $a_{i}=\frac{1}{6}+A\left(\frac{1}{3}\right)^{i}+B\left(-\frac{2}{3}\right)^{i}$ for some constants $A$ and $B$ for $i \geq 1$. Since $a_{1}=0$ and $a_{2}=\frac{1}{3}$, we can solve for $A$ and $B$ and get
$$
a_{i}=\frac{1}{6}+\frac{1}{6}\left(\frac{1}{3}\right)^{i}+\frac{1}{3}\left(-\frac{2}{3}\right)^{i}
$$

The answer to the problem is then
$$
\begin{aligned}
6 \cdot 3^{n-1}\left(a_{n-1}+2 b_{n-1}\right) & =6 \cdot 3^{n-1}\left(a_{n-1}+1-a_{n-1}-3 a_{n}\right) \\
& =6 \cdot 3^{n-1}\left(1-3 a_{n}\right) \\
& =6 \cdot 3^{n-1}\left(1-3\left(\frac{1}{6}+\frac{1}{6}\left(\frac{1}{3}\right)^{n}+\frac{1}{3}\left(-\frac{2}{3}\right)^{n}\right)\right) \\
& =6 \cdot 3^{n-1}\left(\frac{1}{2}-\frac{1}{2}\left(\frac{1}{3}\right)^{n}-\left(-\frac{2}{3}\right)^{n}\right) \\
& =3^{n}+(-2)^{n+1}-1 .
\end{aligned}
$$
30. [16] How many ways are there to color the vertices of a $2 n$-gon with three colors such that no vertex has the same color as its either of its two neighbors or the vertex directly across from it?