River -
From what you've described, I suspect that Wednesday will turn light blue or orange when you look at it tomorrow, Monday. Wednesday and Thursday will be light blue when you look at them on Tuesday, and Wednesday, Thursday, Friday will be light blue if you look at them on Wednesday. Sounds like you're looking at a shop that can't be done on Mondays or Tuesdays or you're looking at a shop that doesn't start till Wednesday.
As for Tuesday, it may look like you can select a grey date and get yourself into trouble, but you can't really select it.
Pop some other shops into your cart and poke around their calendars. Just don't confirm them if you select any dates.
The light blue dates are your current window. If you're just starting out with Cori, then probably all of this month's shops have blue showing for today, tomorrow and the next day. You could sign up for one of those 3 days, and you would need to complete it on that *specific* day.
An exception to this rule would be shops that can only be completed on certain days of the week. Any day that is not acceptable due to client standards will be grey, even if that day is today, tomorrow or the next day.
Beyond that three day light blue window, you see green days. That means that the location can be shopped on those days, once they're within the three day window. If today is Sunday, Wednesday is the first green date, but it will be light blue when you look at it tomorrow, which means that you could sign up for it tomorrow.
Red days mean that you cannot shop that location on that day at all. It won't turn light blue once it gets into the today-tomorrow-next day window. This is because either you have shopped it recently and need to wait a while or because that shop cannot be completed on that day by anyone (usually because it's beyond that shop's deadline.)
Finally, you may see an orange day. That's the first date on which you can sign up for a shop, usually meaning that when I'm in my 45 day re-shop waiting period, on the orange day I could go to the board and sign up for the blue dated shop (which at that point will be two days after the orange, because you can't ever sign up more than two days in advance.)
Except that you can - for next month's shops. Cori makes it possible for you to sign up weeks in advance for the first three days of next month. But the price you'll get for those shops is a pittance and an insult. During or after those first three days, prices will go up for every shop, at different rates and at different intervals. Don't ever, ever, ever take the opening price. You will be mocked and verbally skewered by this forum, and after you see what the prices of the other shops go up to, you will ask for an extra skewer.
Hope this helps.