When painting portraits, from memory, and still life the final product is always far darker than the subject actually is. The number and brightness of indoor lights does nothing to improve the brightness of the painting, and neither does placing the subject outside in direct sunlight. This is an extremely frustrating issue given that the only way to paint something specific is to use the portrait, from memory, or still life options.
@Rainbowliin The painting looks to be to be roughly the same brightness as the walls around the statue that was painted, if somewhat blurrier. Maybe the issue is that the "capture" function is briefly making the subject look brighter than it is, as you highlight the area you want captured. I don't have a lot of experience with this particular way of painting, but you might try capturing some scenery in the middle of the day and compare the painting to how the scenery looks when you're not using the capture tool to highlight it.
Regardless, I'm not sure what anyone here can do to help. TS3 has been out of development for five years, so there's no one at EA to address the issue. You could always ask at Mod the Sims if anyone knows of a painting mod that could help you.