You can find currently two versions of the Outdoor app. The Outdoor 8 app with the blue icon has one big advantage, it can record your track in the background. The green Outdoor App does not have this ability.
This has a technical reason, because of restrictions by WP8.1 vs. WP8.0. If the app is based on WP8.0 internally, it allows the background location tracking. There was no way to re-enable this for the green WP8.1 app, so the blue app was born.
You can unlock the trial version of the blue Outdoor 8 app for free if you have bought the green app already. In the green App go to Info / Apps and press the unlock button. You must install the blue trial version before. If you had bought the Geocaching-InApp, the Geocaching features will be unlocked in the blue App too.
If you plan to buy the Outdoor App, take the blue one.