An easy way can be to have different UIImageView's containing a colored-triangle, then selecting the different image to display according to certain values/preference that you setup. Taken from here:Drawing a triangle in UIView
You will need to do some math to calculate the correct points, but this is the general way of drawing a triangle. You can just create your own subclass of UIView (call it CornerTriangle) and then add it to the cell while settings its color to match your needs.
-(void)drawRect:(CGRect)rect
{
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextBeginPath(ctx);
CGContextMoveToPoint (ctx, CGRectGetMinX(rect), CGRectGetMinY(rect)); // top left
CGContextAddLineToPoint(ctx, CGRectGetMaxX(rect), CGRectGetMidY(rect)); // mid right
CGContextAddLineToPoint(ctx, CGRectGetMinX(rect), CGRectGetMaxY(rect)); // bottom left
CGContextClosePath(ctx);
CGContextSetRGBFillColor(ctx, 1, 1, 0, 1);
CGContextFillPath(ctx);
}