I am wondering if I were to purchase tartan from a mill would it be likely that the tartan is already in stock or do they weave it in order to save the yarn used?

If there are any posts that anyone could refer me to I will be very grateful
For example lets say I wanted a certain amount of yards in the Ross Hunting Tartan but substitute the red yarn I wanted orange , yellow, or some other yarn they already have in stock.

If it is the case that a mill weaves a certain amount of yards AFTER someones places an order, then it would seem reasonable to ask for a color switch, with consideration for which colors they have available, without an increase in price based on the idea that the work would not be made anymore complicated.