The key is to use thin embossed card to make the outside covering of the folios. Even without polish it looks far better than smooth paper.
Most craft stores have a selection of thin cards in cloth or pebbled textures, both work well.
Print the covers in full colour with all the lettering and logos as close as you can get them.
Then once the folio is completed and the glue is dry, just apply boot polish, wait a few minutes for it to dry out a little, then buff to a light sheen.
Coloured boot polishes can usually be applied over the logos and lettering without darking them too much, but experimenting is essential. The materials are cheap, so it's not a problem to try a few things.
It's not that hardwearing, so i'm also trying out liquid boot polishes and leather dyes to see if they penetrate deeper and dry to a harder finish, first indications are looking good
Remember it will never be as hardwearing as actual leather or cloth, so it's just for light use or display.
I'll post new stuff as I do it, but i'm having a break just now, i've just did a load of cards for someone and i'm out of steam :lol