Pintura literally means "paint" or "painting." While it's a common word, its use in this song is quite unique and idiomatic.
Melina León sings, "no me miras y no me hablas ni en pintura me quieres ver," which translates to "you don't look at me and you don't talk to me, you don't want to see me even in paint." This phrase, "ni en pintura," is an idiomatic expression meaning "not even in the slightest" or "not even a trace." It dramatically emphasizes how much the other person wants nothing to do with her, making it a memorable and engaging word choice.
Melina León's "Ya No Soy Buena" is a sassy comeback to someone who's suddenly decided she's not good enough. She points out the hypocrisy, saying, "ahora tú dices que no soy buena y ayer decía que era su amor" [now you say I'm not good, and yesterday you said I was your love]. It's clear this person is upset because she's no longer doing what they want or feeling the same way.
She's not holding back, telling them straight up, "no sólo quise que comprendiera que ya no siento pasión por ti" [I only wanted you to understand that I no longer feel passion for you]. She's done pretending, and if that makes her "mala" [bad] in their eyes, then so be it! The song is all about reclaiming her feelings and not letting someone else's disappointment define her worth. She even says, "mi corazón que es que ya no lo pienso el sol o viento que vuelen los sentimientos" [my heart is that I no longer think of the sun or wind that makes feelings fly], showing she's moved on and her feelings have changed.