The only real answer, of course, is what color and style of hose will depend upon your kilt and other aspects of your outfit. There is no stock answer, and I'm not a big fan of assigning particular colors their appropriate times of day or levels of formality.

That being said, I thought this might be a good occasion to revisit a blog entry I did two years ago on the topic of kilt hose:
http://blog.albanach.org/2005/07/on-kilt-hose.html

Also, another option is the Balmoral hose that we are offering at the Scottish Tartans Museum gift shop:


These have a self-colored sock with a custom hand knit basket weave pattern cuff. They are a nice option in-between solid hose and the full argyle. And, I beleive they can be worn equally well for casual and formal wear. While they are much more expensive than standard hose ($150), they are still less than the typical price for argyle hose of the same quality, which can easily be over $200.

Just another option to consider!

Then there is always the traditional cadadh. You'll have to make your own, but these will have the same effect as the argyle hose (they were the inspiration for the argyle hose, after all), but cost a lot less. Even though they are an historic style of hose, they work just as well for modern dress, even formal wear.