So long as
Ford said, you can have any color of car so long as it is black. Gives a flavor of the supply led state of market. Apart from any technical difficulties of producing different color car,Ford did not have to worry about customers' color preference.

How to explain so long as here. Thanks

Question

So long as
Ford said, you can have any color of car so long as it is black. Gives a flavor of the supply led state of market. Apart from any technical difficulties of producing different color car,Ford did not have to worry about customers' color preference.

How to explain so long as here. Thanks

Answer

"so long as" can be translated "only if" here. 
In this example, the speaker is making a joke playfully saying that you can choose any color, only if the color you choose is black (meaning you actually don't have a choice at all).

hope this helped :)