Free Paint Mode is feature in de Blob that allows players to replay any of the 9 levels in the game. It should be noted that Lake Raydia is omitted from this mode as explained later.
In this mode, there is no time limit or Inkies present. All of the sector gates and transformation engines are unlocked. These reasons are why Lake Raydia is not seen as that level houses the final boss, Comrade Black. However, whilst most hazards such as electric plates and spikes are removed, ink is still prevalent in the levels.
To unlock free paint for any level, simply complete the level in story mode. This can be used to practice if you are attempting the 100% completion trophy.