A blue circle means a status code of 2xx, such as 200. This is the normal status code for a regular web page on the Wayback Machine. A blue circle is usually a safe choice.
A green circle signifies a 3xx status code, which means a redirect. Try to avoid the green dots when picking a date to scrape. It's better to get the target URL which the redirect leads to.
Orange means an error with a 4xx status code.
A red dot around the date means a server-side error, which carries a 5xx status code.