I need to know the color-values of a certain pixel in a PNG including the alpha value. But if I use pixelmator's color picker, the alpha value is not available.
Where can I find that information?
I don't believe it's possible to use the eye dropper in Pixelmator to find this out (tested in Pixelmator 2.02 with Lion).
You could sample the colour, then create a new layer and paint a small square next to the colour you're trying to match, then adjust the opacity of the layer until they match perfectly. That would require a bit of patience though.
Acorn's eye dropper does pick up opacity and colour, so that's an option if you own it.
Photoshop's eye dropper doesn't pick up opacity, but it's possible to see the opacity in the info panel (if you have it set up to show opacity). You can also command-click a layer's thumbnail in Photoshop, then save the selection and interrogate the channel it saved, if you wanted to check out the entire alpha channel.
This question has been baking my noodle for some time and I have done some experimenting in Photoshop. Perhaps it isn't too different in Pixelmator. This is my solution:
I duplicated the transparent Layer till the transparency was entirely gone. Then I measure the pixel again with its now darker and non-transparent value.
Then I filled a new layer in the resulting color and slowly reduced the Opacity till they matched. And the result was pretty damn close.
Hope that helps. I can post a screenshot if you'd like or if something is unclear.