The phrase "brighten it up" is correct and usable in written English.
You can use it whenever you want to encourage someone to make something more cheerful or positive. For example: "Let's try to brighten it up by adding some colorful decorations to the room."
When the wall color is dark, "you need something to brighten it up," she said.
"Duveen would unquestionably have ripped off the frame," he said, "and sent the picture to somebody to brighten it up".
It might be that a busy, bold pattern in vibrant colours is just what your bedroom needs to brighten it up for the winter mornings ahead.
A week after Michael died they asked me to whitewash his cell, saying they had to brighten it up for the next person.
("That's a nice, easy high C, sopranos — I noticed last week you've been singing this well". "More energy, altos — brighten it up").
She happily tosses an extra stick of butter into one day's tomato soup to "brighten it up a bit" because the sailors, some of whom are barely out of their teens, work long hours and at mealtimes they are starved.
But music really brightened it up".
In the Caribbean, she found a man who had built an new island from rubbish and brightened it up with papaya and coconuts.
Mr. Arthur Sullivan has entered with heart into the spirit of Mr. Gilbert's fun, he has brightened it up with the most fanciful and delightful music".
The day dawned overcast and cloudy but we hoped that it would brighten up - sadly it didn't and we never got to see the sun but what we did experience was magical.
"According to our estimations, the Chelyabinski meteor started to brighten up when it was between 32 and 47 km up in the atmosphere," they write in their paper, which has been posted to the online astronomy preprint site ArXiv.org.
