Count number of ways to paint a fence with N posts using K colors
1. Code review


The question says, "Write an algorithm that counts the number of ways" but the program does not only do this: it also builds a list of the combinations. This means that the program is limited to values of \$N\$ and \$K\$ that are small enough that the list of combinations fits in the storage space of the computer. Whereas a program that did not have to produce this list could work for much larger values of \$N\$ and \$K\$, as we'll see in §3 below.
It's good practice to write a docstring for each function, describing what the function does, the arguments that it takes, and what it returns.
If a function returns a list, it does not also need to return the length of the list. If the caller needs that information, it can call len just as easily. So the function could just return combos_list.
In the line:

combos_list = new_combo_list[:]


it's unnecessary (and wasteful) to take a copy of new_combo_list. The new list can just be assigned to the old name, like this:

combos_list = new_combo_list

When a loop index (here i) is not used, it's conventional to name it _.
The code that's guarded by the line:

if not combos_list:


only runs once (on the first iteration of the loop), so it would be simpler to move it out of the loop, have one fewer iteration of the loop, and so avoid the if ... else .... Like this:

combos_list = []
for c in colours:
    combos_list.append([c])
for _ in range(1, n):
    new_combo_list = []
    # etc.

Instead of:

combos_list = []
for c in colours:
    combos_list.append([c])


use a list comprehension and write:

combos_list = [[c] for c in colours]

These lines:

combo_to_add = combo[:]
combo_to_add.append(c)
new_combo_list.append(combo_to_add)


could be simplified to:

new_combo_list.append(combo + [c])

This condition:

if not combo[len(combo) - 2:] == [c, c]:


could be simplified to:

if combo[-2:] != [c, c]:


(since Python allows you index a list backwards from the end using negative numbers).


2. Revised code

def find_combinations(n, colours):
    """Return a list of combinations of n items from the sequence colours,
    with the constraint that no colour appears three times in a row.

    &gt;&gt;&gt; [''.join(combo) for combo in find_combinations(3, 'ab')]
    ['aab', 'aba', 'abb', 'baa', 'bab', 'bba']

    """
    combos = [[c] for c in colours]

    for _ in range(1, n):
        new_combos = []
        for combo in combos:
            for c in colours:
                # Check that last two colours aren't both c.
                if combo[-2:] != [c, c]:
                    new_combos.append(combo + [c])
        combos = new_combos

    return combos


Note the example code in the docstring. This can be run as a test case using the doctest module.

3. Let me count the ways

The question asked for "an algorithm that counts the number of ways", so here's how I'd go about that.

I'll start with some notation. Assume that \$k\$ is fixed. Then:


\$T(n)\$ is the total number of ways to paint \$n\$ fenceposts with \$k\$ colours, subject to the constraint that no three adjacent posts are the same colour.
\$D(n)\$ is the number of ways to paint \$n\$ fenceposts with \$k\$ colours such that the last two fenceposts are different colours (or \$n &lt; 2\$). 
\$S(n)\$ is the number of ways to paint \$n\$ fenceposts with \$k\$ colours such that the last two fenceposts are the same colour.


And then the following recurrence relations are clear:


\$D(n) = (k−1)(D(n−1) + S(n−1))\$ for \$n ≥ 2\$
\$S(n) = D(n−1)\$ for \$n ≥ 2\$
\$T(n) = D(n) + S(n)\$


We can use (2) to eliminate \$S\$ (being careful about the bound on \$n\$ as we do so), getting the recurrences:


\$D(n) = (k−1)(D(n−1) + D(n−2))\$ for \$n ≥ 3\$
\$T(n) = D(n) + D(n−1)\$ for \$n ≥ 2\$


And then substitute (5) into (4), getting:


\$D(n) = (k−1)(T(n−1))\$ for \$n ≥ 3\$


and so:


\$T(n) = (k−1)(T(n−1) + T(n−2))\$ for \$n ≥ 3\$


Since the recurrence depends only on the last two values of the sequence, it's easy to program:

def triple_free_combinations(n, k):
    """Return the number of ways to choose n items (with k choices for
    each item), subject to the constraint that no colour appears three
    times in a row.

    """
    if n == 0:
        return 1
    a, b = k, k * k
    for _ in range(n - 1):
        a, b = b, (k - 1) * (a + b)
    return a


(You should recognize this as a very simple example of the tabular method, aka "dynamic programming".)

The effort that went into find_combinations is not wasted, because now we can use that function as source of test cases:

&gt;&gt;&gt; from itertools import product
&gt;&gt;&gt; all(triple_free_combinations(n, k)
...     == len(find_combinations(n, list(range(k))))
...     for n, k in product(range(1, 8), repeat=2))
True


Another useful source of test cases is the On-Line Encyclopedia of Integer Sequences. For example, when \$k=2\$ we have sequence A128588:

&gt;&gt;&gt; [triple_free_combinations(i, 2) for i in range(16)]
[1, 2, 4, 6, 10, 16, 26, 42, 68, 110, 178, 288, 466, 754, 1220, 1974]


and similarly when \$k=3,4,5,\ldots\$ we have A121907, A123620, A123871, ....

Finally, note that triple_free_combinations is capable of computing values that are far beyond the reach of the original function:

&gt;&gt;&gt; triple_free_combinations(40, 40)
472189463994618801762969606024644975976973566598588071193029590400