I really am just starting to learn processing.  I copied one of the earliest examples from the tutorial and varied the parameters.  It worked nicely but I couldn't get the line color to change--it was always black.  why is that?  Here's the program:
size (500, 500);
background (255, 0, 0);
line (15, 15, 400, 15);
stroke (255, 255, 255);

I thought that this should make a white line, but it's black.  Why?