Vampix provides a quick and easy way to create eye-catching colour splash-type effects.
It turns every part of an image to black and white, apart from the colour you select: blue eyes, a red rose, whatever it might be.
Load your target image and the program displays it alongside the preview window, which initially is just a greyscale version of the original.
Click on a colour within the original image, though, and the preview updates to show every pixel of that shade. If you've clicked on green trees in the background of a landscape scene, for instance, all similar pixels in the preview image will appear green, while everything else stays grey.
If this doesn't look quite right, clicking on a fractionally different shade updates the preview immediately. You can fine-tune the results by adjusting a threshold, too. Decreasing the threshold matches colours more accurately to your selection, displaying less colour overall; increasing the threshold extends the tolerance and gives you more of the original shades.
This works, but can be quite limited. You can only select a single colour, and there's no way to localise the effect. If you click on a blue sky, for example, any other pixel of that shade - water, clothing, whatever - will also be coloured in the finished image.
Still, this simplicity does keep the program easy-to-use, and it can deliver good results with the right image.
Vampix is extremely basic - more suited to being a single effect in an image editor, than a stand-alone program - but it's also easy to use, and can produce eye-catching results in some situations. If you need this effect and are looking for something simple, give it a try.