SVG and CSS
Recently someone mentioned that they knew someone else who had just brought a duvet cover that has "Huge purple flowers" on it. This gave me a most interesting mental picture - and I recreated in Inkscape (which has just been updated!) for you to see - the result is shown above. Note that those plants would be at least 3 times your size.
The square above is actually black and set to have a low opacity, to it sets on the colour anything that is behind it. It is also done in percentages, so you can set the width and height of the containing
<img /> tag and it should scale accordingly.
I might do some more of this in the future - it looks to me like a wonderful way to create a nice little self contained widget that you can use anywhere.