pic5(whitecirc)

import geometry;

size(9cm,0);
defaultpen(fontsize(12pt));
dotfactor = 4;

fill(box( (-1,-1), (0,0) ), blue);
fill(box( (1,1), (0,0) ), green);
fill(box( (1,-1), (0,0) ), red);
fill(box( (-1,1), (0,0) ), yellow);

for(real i = 0.1; i<1; i+=0.1){
fill( circle(origin, i), white+opacity(0.5-(i/3)) );
}