В методе drawRect:
выполните:
- (void)drawRect:(CGRect)rect
{
CGContextRef ctx = UIGraphicsGetCurrentContext();
UIGraphicsPushContext(ctx);
CGContextSetRGBFillColor(ctx, 1.0f, 1.0f, 1.0f, 1.0f); // white color
CGContextFillEllipseInRect(ctx, CGRectMake(10.0f, 10.0f, 100.0f, 100.0f)); // a white filled circle with a diameter of 100 pixels, centered in (60, 60)
UIGraphicsPopContext();
}