It may be due to a lack of sleep (again) but i can´t find the option in the user preferences can someone help me out please?
Reason?
I usually set the background color of my IrfanView, Image editing tools or Renderwindows to 255,0,255 or 0,255,0 for the simple reason to trace transparency quickly…
Just to clearify things in the first place, i dont mean the horizon, zenit or ambient color… really simply the backtroundcolor of the render window 
Am I crazy, or isn’t that just the world color you’re looking for?
hmm no its exactly not what i am looking for.
The world colors also change the results of ambient occlusion, ambient of the material and so on.
Make a render, and then resize the render window… you see that the render window has a backroundcolor.
What i want to be more specifc:
lets say i render a planet. space: horizon zenit and ambient are black. space is dark.
now i render the planet with premul or key, to finally have a png with the planet only with transparent background.
Blender now shows up the render with the black from the world settings, eventhough its transparent when i save the png.
I want to change the background of the renderwindow and when i render i want to see the planet only and the rest of the renderwindow should be filled with the pink-ish or green background… else i always have to save the image, open in a picture viewer and check the transparency.
The actual colour that you see when you enlarge the render window is hard coded as far as I am aware (see line 340 of renderwin.c). The closest thing I can think of to what you’re talking about is the background image option. In Render Buttons, under Output, you can set an image to use as the background. The colours from the world settings will still be used of AO, etc., but the image will be shown instead when rendering. Note that this only works with Sky, not Premul or Key. If hot pink is your thing, just make a hot pink image the same size as you’re rendering. =)
what i was afraid of ^^
Would be a nice feature to see the alpha of the finished still already in the render window and be able to set a custom background color… blenderstorm…
I’m not sure if I misunderstood. The above method lets you see the alpha on a custom background.
See the attached file for an example: monkey with no material, no lights, Blue Horizon, Red Ambient, AO.
I get it. That grey background…I see…
pressing A in the render window once the render is finished will show the alpha.
And to save a png with the alpha you need to select the RGBA setting in the render settings, right under the dropdown menu to select the file type (png, jpg… etc…)
日本語 Koryo. I used that one before… i think i am suffering burnout by now ^^ i didn´t think of that one.
and thanks Lumpycow you made my day that was something i didn´t know. It´s not really design feedback but the technical feedback of the alpha channel i need…