Paku and PurpleAir should closely match, as long as you have the same conversion settings on both the app and the website. Tap the gear to see your conversion settings. Note that Paku does not offer all the options that PurpleAir does.
First, make sure your sensor is publicly visible, as Paku only displays public sensors. The app will learn about new sensors by refreshing once a week, since fetching the complete list of sensors is a large amount of data.
If you recently added a sensor to the network, you can force the app to manually refresh in the settings using the “Refresh Sensors” option.
Sensors will show up as grey if they haven't reported new data recently, or if the data being reported has a low confidence - that is, it may be inaccurate. These sensors will not show up in widgets either - the nearest accurate sensor is automatically chosen when using the “Nearby” setting.