A couple ways
Sure, you can do this. There are a couple of ways
1. Use one of the selection tools to select your daughter's eyes. Note that you can select one eye and then select the other by putting the selection tool in the mode that adds to the selection.
2. Invert the selection. (Shift + CMD + I)
3. Desaturate the image (Option + CMD + U). It will only desaturate the part that is selected, so your daughter's eyes will retain their color.
Voila!
If you want to use Aperture's more sophisticated Monochrome Mixer to generate a custom black and white image, you can:
1 Create the black and white version in Aperture
2 Export both the black and white versions as jpg or tiff files
3 Open both files in Pixelmator
4 Copy all of one image and paste it onto the other as a second layer (I haven't found a way to open a second image as a layer directly in Pixelmator)
5 Make sure the black and white layer is on top
6 Create a layer mask (if you open the layer dialog, you can right click on the image of the layer and select add layer mask)
7 Select the layer mask by clicking on it (it will be just to the right of the layer in the layer dialog)
8 Make sure white is selected as the foreground color on your tools
9 Use the pencil or paintbrush to paint on the layer mask. It will look like you're painting on the image, but the spots that you draw on will become transparent for that layer and the color layer will show through.
If you're not familiar with these tools this may sound really complex, but it's not too hard. Let me know how it goes!
