The friendship between the two are sort of a back and forth thing. They are frenemies, basically. Lightbulb and Paintbrush used to have a slight friendship, sharing a few bonding moments, before Lightbulb's actions finally caused Paintbrush to snap in Cooking for the Grater Good. Paintbrush declares him/herself as captain and tries to lead the team. When Lightbulb makes her cookie pizza which ends up winning for their team, Paintbrush is still slightly unimpressed.
However, in A Kick in the Right Direction, Paintbrush tries to get on Lightbulb's good side, only for Lightbulb to be oblivious when another team member acts offensive. Despite being angry, Paintbrush says nothing when being sent to the calm-down corner. Lightbulb is oblivious to the hate Paintbrush is giving her, and she acts as if the two are friends (making this a one-sided conflict).
In Let 'Er R.I.P., the two are seen talking in the haunted house, Lightbulb does a hideous rap, then switches herself on. Paintbrush first gives a surprised reaction, only to fix it with an ordinary response, and insulting Bow in the process. She flies into Paintbrush's bristles and drags Paintbrush around in the air, to which Lightbulb assumes Paintbrush is flying, exclaiming they can be "flying buddies". Then Lightbulb grabs on to Paintbrush's back and they both fly out of the mansion.
In Everything's A-OJ, Lightbulb is the only Bright Light that offers to help Paintbrush, which Paintbrush was thankful for, but Lightbulb trips and shatters right after. Paintbrush's look at Lightbulb's shattered body showed horror and concern.
In Rain On Your Charade, Lightbulb is joking about finding Marshmallow's bottle cap collection, while Paintbrush is holding up a sign saying "FIND THE CAPS OR DEATH".