Solo Stickers can be earned only by winning in solo modes (i.e. Score Rally, Perfect Heist & Cosmic Rush).
Collecting these elusive cyan stickers may take some time, however, the rewards are great and definitely worth the challenge :)
Also, you won’t get any duplicates in these albums** so you will complete them much faster!
Please note that if your Solo album is complete, you will have the ability to reset the album, giving you the chance to collect the stickers and rewards from the beginning.