What do the colored P, B, U, G, S, 6, W flags mean?

These show some properties/services the device offers:

• P - pingable (green): the device responds to ICMP ping requests
• B - Bonjour (brown): the device provides Bonjour services
• U - UPNP/DLNA (blue): the device provides UPNP/DLNA services
• G - gateway (red): the device is a gateway
• S - scanning device (yellow): the device from which you perform the scan
• 6 - IPv6 (purple): there are known IPv6 addresses for the device
• W - web interface available (cyan): port 80 or 443 open

In the paid version of the app it is also possible to filter the scan result using the flags. After selecting the magnifying glass icon for filtering from the toolbar, you can prefix the flag name with # to show only devices with the particular flag set. So for instance after entering #P you will only see pingable devices.