Https://youtu.be/C6KxsmxWeoY​

It is clear that the purple flower color allele is dominant and the parents are heterozygous.

When there are three dominants and one recessive flower, the parents would need to be heterozygous, Bb. If you draw the punnet square, it would turn out like this.

          B         b

    B    BB     Bb

    b     Bb     bb

Hope this helps!!!