At Tobita, at least, I think the day of the week matters more than the month of the year (outside of standard holidays, obviously). I would avoid Friday evenings as hordes of young salary-dicks descend upon the place walking up and down in packs and taking up all the good ones. Saturday probably has the best selection, and it's best to hit early. I know guys who swear by going at 3pm and beginning to prowl, but I've always found that, IN GENERAL, it's the lesser desirable girls who open up that early, and those are usually at the end of the block near the underpass street. I'd recommend starting to pace the streets around 4:30pm and keep going for an hour.
I've never noticed many of the dark/tanned types but Tobita will always have at least one of every type--nurse, geisha, cheerleader, dork, fat, old, model pretty, homely as fuck, etc. The key thing, no matter how desirable the girl is to you--especially if you aren't East Asian--is to gauge the reaction. If the girl is eyeing you flirtatiously and better yet waving at you, hit that shit. Otherwise, no matter how hot she is, if the girl looks uninterested or, worse, is avoiding eye contact, don't go for it because even if they mamasan lets you in you're going to have a very shitty experience.
I must add, though, if your thing is just dark/tanned, have you ever considered hitting up the Filipino clubs? At least the brown skin will be genuine and the girls will probably be far cheaper, even if you have to pay some lady under the table at a hostess club. There were a bunch of Filipino clubs in Kobe-Sannomiya area and a ton in the Kyobashi-Temma-Umeda shotengai region back in my day. The fun thing about Flipinos is the women, even if you're paying them for it, are usually horny as fucking rats. The downside is that they've got the emotional and temperamental stability of Koreans but instead of just trashing your flat and flailing around aimlessly they actually will try to physically harm you. I loved dating Filipino girls as a teen. That said, sprinting to my car while a half-naked, shrieking brown woman is chasing you with a kitchen knife can be entertaining when you're 19 but not so much once you've got a mortgage and can't just trade neighbors.