Most Scottish Basket-Hilt replicas come with a liner in the basket, usually red. I've seen photos of antiques with these liners as well, but from what I've seen, most reenactors remove them.

How common were basket liners in Scottish swords during the 17th-18th centuries? Also, though red seems to be the most common, what other colors were in use in basket liners?

One more thing, is there any practical purpose of the liner, or is is purely aesthetic?

Basket liners were originally smaller, not the "full basket" things from more recent times. They were made of leather, and were intended as "buffers" between hand and basket, and as such did have a practical purpose. Later liners were still made of buff leather, but were covered in other material -- usually red cloth of some kind (the legend has it that the red cloth was made from the "redcoats" of British soldiers -- there is no real proof for this, of course). The liner grew over time, reaching the "full basket" dimensions currently known, and as the sword became more a symbol and less a weapon for actual use, the leather was dispensed with, leaving just the red cloth. It was by that time entirely aesthetic.

Armour Class makes baskets with "historically accurate" liners. Here's one of the earlier ones I was talking about:
http://www.armourclass.co.uk/Data/Pages/Scottish_10.1.htm

And here's a somewhat later version, still made of leather covered in cloth:
http://www.armourclass.co.uk/Data/Pages/Scottish_2.htm

The liners are removed for various reasons. On many originals, naturally time is not very friendly to the material, causing the liners to degrade, sometimes they just fall apart, sometimes they are removed by collectors during cleaning/restoration.

Thank you!

Do you by any chance have instructions or a pattern for making a historically accurate one? I'm considering replacing the liner in my basket-hilt.

Nope, sorry....