Get some brown boot polish (preferably kiwi, but cherry blossom is just as good) and a couple of boot brushes, one for on and one for off. Clean all the muck and mud off your boots. Get your on brush and cover it in a fair amount of polish, brush all up and down the boot and leave it for about 5 mins (don't let the polish dry though). Get your off brush, take all the polish off and your done! If you want you can have a go at bulling the toe caps with wet a polishing cloth/bit of cotton wool, put some polish on it and go over the toe caps in small circles. After a fair bit of time and effort you'll get that lovely mirror shine.
It can take a while and seem a bit boring but it's very satisfying once it's done. Hope this helps 
Dark tan should be OK. You might give it a once over with Ox Blood from time to time to build up a bit of patina and depth of colour.
If these aren't prone to falling apart I would just stick with them rather than spend again on genuine Corcorans. Modern Corcoran boots while very well made are coated with a very plastic finish over the leather which doesn't absorb polish too well and scratches to reveal a light coloured leather beneath. It can be removed with paint stripper but that's a very messy job and the leather underneath needs to be re-dyed.
