I would like to know how can I change color of RGBA image when alpha channel will be unchanged for each pixel. If I use Paint Box Tool it will set alpha channel for all pixels to constant value.
Look at picture:
There's an option somewhere on the tools pallet called 'preserve transparency' that should do exactly what you are looking for.
The fast way to accomplish this, rather than using the Paint Bucket, is to use the keyboard shortcuts.
To fill with color while preserving transparency hold down Shift while you fill using:
I don't remember when I last used the Paint Bucket. It's been so long I'd forgotten it was there...
Although not as slick as Alan's method, which I will be using after this, a general method is to lock layer transparency before fill, painting, filters, etc.
All the current answers are probably better solutions in most cases, but in some cases you can also:
Using a clipping/layer mask—this can be done in a few ways:
Use Previous Layer to Create Clipping Maskoption.
Layer > Layer Mask > Reveal Selection.
It's a slightly more roundabout way to achieve the same general thing, but clipping/layer masks let you do some cool things sometimes, like just painting or pasting images into the layer and still preserve the alpha channel.