The mechanism is documented here. Basically you can access the constructor by the following syntax:
js> new java.awt.Color['(int,int,int)'](1,2,3); // no spaces allowed!
java.awt.Color[r=1,g=2,b=3]
or
js> java.awt.Color['(java.awt.color.ColorSpace,float[],float)']
function <init>(java.awt.color.ColorSpace,float[],float)() {
[native code, arity=0]
}