Simple NSGradient usage.

gistfile1.h

- (void)drawRect:(NSRect)rect
{
    NSRect bounds = [self bounds];
    NSBezierPath *clipShape = [NSBezierPath bezierPath];
    [clipShape appendBezierPathWithRoundedRect:bounds xRadius:40 yRadius:30];
    NSGradient *aGradient = [[[NSGradient alloc]
                              initWithColorsAndLocations:[NSColor redColor], (CGFloat)0.0,
                              [NSColor orangeColor], (CGFloat)0.166,
                              [NSColor yellowColor], (CGFloat)0.33,
                              [NSColor greenColor], (CGFloat)0.5,
                              [NSColor blueColor], (CGFloat)0.75,
                              [NSColor purpleColor], (CGFloat)1.0,
                              nil] autorelease];
    [aGradient drawInBezierPath:clipShape angle:0.0];
}