The first thing that popped into my mind was "Mandelbrot!".
Yes, I know there already is a mandelbrot submission. After confirming that I'm able to get it below 140 characters myself, I have taken the tricks and optimizations from that solution into mine (thanks Martin and Todd). That left space to choose an interesting location and zoom, as well as a nice color theme:
unsigned char RD(int i,int j){
double a=0,b=0,c,d,n=0;
while((c=a*a)+(d=b*b)<4&&n++<880)
{b=2*a*b+j*8e-9-.645411;a=c-d+i*8e-9+.356888;}
return 255*pow((n-80)/800,3.);
}
unsigned char GR(int i,int j){
double a=0,b=0,c,d,n=0;
while((c=a*a)+(d=b*b)<4&&n++<880)
{b=2*a*b+j*8e-9-.645411;a=c-d+i*8e-9+.356888;}
return 255*pow((n-80)/800,.7);
}
unsigned char BL(int i,int j){
double a=0,b=0,c,d,n=0;
while((c=a*a)+(d=b*b)<4&&n++<880)
{b=2*a*b+j*8e-9-.645411;a=c-d+i*8e-9+.356888;}
return 255*pow((n-80)/800,.5);
}