You can combine different shirt colors in an order but the imprint color must remain the same for all the shirts.
While we use the same ink for the entire press run, each shirt color reacts differently with the ink. Some people will see the ink colors as changed based on the differences of the fabric colors. This is especially noticable with light ink on dark shirts.
Another issue is contrast. White ink, for example, will stand out and be very visible on a dark color shirt but will be barely noticeable on an Ash color shirt. Choose the shirt colors carefully to achieve the results you are seeking.