image processing - iOS: Applying a RGB filter to a greyscale PNG

Question

I have a greyscale gem top view.

(PNG format, so has alpha component)

I would like to create 12 small size buttons, each in a different colour, from this image.

For the sake of tidiness, I would like to do this within the code rather than externally in some art package.

Can anyone provide a method (or even some code) for doing this?

PS I am aware of how to do it in GL using a ridiculous amount of code, I'm hoping there is a simpler way using core graphics / core animation

EDIT: Working solution, thanks to awesomeness from below answer

    CGSize targetSize = (CGSize){100,100};
    UIImage* image;
    {
        CGRect rect = (CGRect){ .size = targetSize };

        UIGraphicsBeginImageContext( targetSize );
        {
            CGContextRef X = UIGraphicsGetCurrentContext();

            UIImage* uiGem = [UIImage imageNamed: @"GemTop_Dull.png"];

            // draw gem
            [uiGem drawInRect: rect];

            // overlay a red rectangle
            CGContextSetBlendMode( X, kCGBlendModeColor ) ;
            CGContextSetRGBFillColor ( X,  0.9, 0, 0,  1 );
            CGContextFillRect ( X, rect );

            // redraw gem 
            [uiGem drawInRect: rect
                    blendMode: kCGBlendModeDestinationIn
                        alpha: 1. ];

            image = UIGraphicsGetImageFromCurrentImageContext();
        }
        UIGraphicsEndImageContext();
    }

Answer 1

The easiest way to do it is to draw the image into an RGB-colorspaced CGBitmapContext, use CGContextSetBlendMode to set kCGBlendModeColor, and then draw over it with a solid color (e.g. with CGContextFillRect).