Converting String to MovieClip

hi every body I’m new in this forum and I need some help in as3
the problem:
I want to convert my string variabale to Movie Clip
like this :
var a:String=“blue”;
I want to turn the value of (a) variable to MovieClip
I mean I want it to be like this :
var blue:Movie Clip
I tested some ways like this but didn’t response

var b:MovieClip=MovieClip(a)

help plz
tnx