The ''It threw some rose petals in the air'' was Magic Leaf.
Roselia throw some petals with different colors to the air, then it comes down and then launches to your Pokemon.
In Gen IV, if a shiny Pokemon appears it will appear stars around it, like this:
Another way (don't know if this works, because I don't have my Pokemon Platinum now) is going to the forms of Roselia and see if it was a ''shiny form''. This can be done in Gen V however I don't know if this can be done in Gen IV.
As you say ''It threw some rose petals in the air'' it is the same was Magic Leaf and it can be learned by Roselia at level 19.
Roselia can be found at Route 208, Route 209, Route 210 South, Route 212 North, Route 221, Route 224, Route 229, Route 230, Trophy Garden and the minimum level is 19 in Route 208.
Like Fated Fathom ♠ said, it is possible that it can be shiny but in this case I'm very sure that it WAS NOT SHINY because of the Magic Leaf.
Hope this helps!