In the movie Selena, a white rose is thrown to her but she doesn’t catch it. It is used as a metaphor to show that she died because once it falls she stops singing as if everything that her and the rose have in common is gone.
One of her most popular songs, Como una Flor, is based off of plastic white roses Selena y Los Dinos would sell at their concerts.
Selena loved white roses. When she passed away, her brother told her fans to bring white roses. And when everyone came to say their goodbyes to her, there was a sea of thousands and thousands of white roses. So that just became like a symbol that’s associated with Selena – the white rose. (See Interview: Suzette Quintanilla - Sister of Selena, the "Queen of Tejano Music")
Of interest, discover 12 Things You Didn't Know About the Movie "Selena", featured at Cosmopolitan.com.