The ratio of marbles of color x, y, and z in a bag is x:y = 1:2 and y:z = 1:3. If there are a total of 27 marbles, how many marbles are color z?

To combine these ratios, we need to make the value for $y$ consistent.
The ratio $x:y = 1:2$ means for every 1 part of $x$, there are 2 parts of $y$.
The ratio $y:z = 1:3$ means for every 1 part of $y$, there are 3 parts of $z$.
To make $y$ consistent, we can multiply the second ratio by 2:
$y:z = (1 \times 2) : (3 \times 2) = 2:6$.
Now we have $x:y = 1:2$ and $y:z = 2:6$.
So, the combined ratio is $x:y:z = 1:2:6$.

Let the common multiplier be $k$.
Number of $x$ marbles $= 1k$
Number of $y$ marbles $= 2k$
Number of $z$ marbles $= 6k$

The total number of marbles is 27.

The number of $z$ marbles is $6k$.
Number of $z$ marbles $= 6 \times 3 = 18$.

The number of $z$ marbles in the bag is $\boxed{18}$.