We've got an email dialog going with you now so we'll continue the troubleshooting there. But to those following along:
The limitation on receiving the weather picture is entirely dependent on the order and time that it takes the ground stations to beam it up. Necessarily, this is done as a continuous stream that more or less repeats with both the graphical (nexrad) and textual (metars/tafs/etc) updates (this is an oversimplification). So, depending on what the system was sending the moment before you powered up and gained reception, the time it takes to get various bits of data can differ from flight to flight. The system IS set up to give you the most detail about the area that you're closest to. But know that a full refresh of all of the TIS-B weather data can take I believe up to 15 minutes to get to you in normal operation when you have continuous reception.
Technically, the purple means that the SkyView has not received NEXRAD data for that area yet, for whatever reason -- be it 1) the receiver is offline, 2) towers are out of range, 3) data just hasn't been beamed up yet, OR that the data for that sector has simply expired (this is getting a bit nitty gritty). Normally this is just because you're waiting for the data in that area of the country.