I assume that the non Scottish kilt companies copied what they saw the Scottish kilt companies providing. If you go to the majority of kilt makers in Scotland they will more than likely offer the off white hose to go with the kilt.

Although these days, if you are younger, they will most likely offer a black shirt and black hose option. And these are kilts which cost $600+ so its not simply to save money. If you go to a Scottish wedding, where the majority of kilts are owned and not hired, you will still see the majority wearing off white hose.

My thoughts on this are that 1. white is the easier option for kilt sales shops, trying to match coloured hose to a kilt is very much a matter of personal taste 2. coloured hose used to be worn by the older generation and the off white hose provides a more modern look and 3. white will go with all occasions, formal or informal, certain coloured hose would not look good for formal kilt attire.

I have several coloured hose and pick the ones which best suit the occasion, the colours and type of attire I'm wearing.