Paint jobs are unlocked by completing challenges with specific bikes. Note that many are tied to night Free Rider challenges- the night option for a trail is unlocked by completing that trail's standard Free Rider challenge.
This YT playlist shows my "X time or less with Y crashes or fewer" runs on each trail. These runs are far from perfect but will illustrate shortcuts to allow for completion within the required times.
Here is a full list of paint jobs:
GraterhornTrail 1
•2:00 or less (Grasshopper)
•1:50 or less with 7 crashes or fewer (Pacebreaker)
Trail 1 Night
•Free Rider (Grasshopper)
Trail 2
•2:50 or less (Grasshopper)
•2:25 or less (Grasshopper)
•2:15 or less with 7 crashes or fewer (Pacebreaker)
Trail 2 Night
•Free Rider (Javelin)
Trail 3
•1:55 or less with 7 crashes or fewer (Javelin)
Trail 3 Night
•Free Rider (Pacebreaker)
Trail 4
•11 crashes or fewer (Geronimo)
•1:40 or less with 7 crashes or fewer (Boar)
Redmoor PeaksTrail 1
•15 crashes or fewer (Pacebreaker)
•2:55 or less with 8 crashes or fewer (Javelin)
Trail 1 Night
•Free Rider (Trailblazer)
Trail 2
•15 crashes or fewer (Pacebreaker)
•2:50 or less with 9 crashes or fewer (Trailblazer)
Trail 2 Night
•Free Rider (Boar)
Trail 3
•2:20 or less with 7 crashes or fewer (Boar)
Trail 3 Night
•Free Rider (Geronimo)
Trail 4
•15 crashes or fewer (Pacebreaker)
•2:00 or less with 7 crashes or fewer (Geronimo)
Sierra RiveraTrail 1
•12 crashes or fewer (Javelin)
•2:10 or less with 7 crashes or fewer (Trailblazer)
Trail 1 Night
•Free Rider (Grasshopper)
Trail 2
•11 crashes or fewer (Javelin)
•2:20 or less with 7 crashes or fewer (Boar)
Trail 2 Night
•Free Rider (Pacebreaker)
Trail 3
•2:10 or less (Javelin)
•2:00 or less with 5 crashes or fewer (Geronimo)
Trail 3 Night
•Free Rider (Javelin)
Trail 4
•2:35 or less with 8 crashes or fewer (Grasshopper)
Mount RileyTrail 1
•11 crashes or fewer (Trailblazer)
•2:10 or less with 7 crashes or fewer (Boar)
Trail 1 Night
•Free Rider (Trailblazer)
Trail 2
•12 crashes or fewer (Trailblazer)
•2:20 or less with 8 crashes or fewer (Geronimo)
Trail 2 Night
•Free Rider (Boar)
Trail 3
•2;20 or less (Trailblazer)
•2:10 or less with 8 crashes or fewer (Grasshopper)
Trail 3 Night
•Free Rider (Geronimo)
Trail 4
•15 crashes or fewer (Boar)
•2:20 or less with 7 crashes or fewer (Geronimo)