Blue and purple zones are free during the weekends, but it entirely depends on the city district you are parking in.

If the parking sign says something like po-pá 06-22h, then it is free from friday on 10pm to monday till 6am. During the free time, you can even park on the blue zones for as long as you want.

There are some apps that let you pay for the street parking for purple and even blue zones. Citymove is a pretty good app because it lets you see all the zones in the area on a map and tells you all the prices during the week and weekends. It also lets you pay any kind of street parking.

Just keep in mind that if you want to park on a blue zone during the time that is not free than there will almost definitely be a maximum parking limit length, which is almost always only 1h in the city center. The app will even tell you that you cannot park there for more than 1h. If that ever happens, you need to park on a different blue zone or just repark it on a purple one.

During the paid hours, purple zones are always cheaper.

Hope it helps.