Blender is a bit limited as an image editor, you may find using an external image editor easier. One option is to use the compositor were masking nodes can be used to select a colour to use as a mask for other nodes.

As your asking with the tag of python, you can access the raw image data through bpy.data.images['img_name'].pixels[x]. This is a list of floats that makeup the pixel data, there are 4 per image pixel (red,green,blue,alpha). You can get the width and height through bpy.data.images['img_name'].size.

for p in bpy.data.images['img_name'].pixels:
    if p &lt; 0.45 and p &gt; 0.48:
        p = 0.2


Change image levels quickly
I have an e-mail server powered by Postfix. Aside from traditional e-mail accounts on my server, I want to maintain some accounts whose only role is to forward all incoming mail to differnet address on different mail system.

For example. I own domain example.com and I set account with address username@example.com on my server. And I want this one account to forward all incoming messages to mailbox in different mail system, in domain that does not belong to me username@stranger.net.
I know, that i can add an entry in in my virtual_alias_maps lookup table.

username@example.com   username@stranger.net


But I'm worried that to deliver message, Postfix will use original envelope sender address. It's okay if sender is from domain example.com that belongs to me, but otherwise stranger.net server may classify this message as a spam.  

Is there some way, to alter envelope sender on delivery ?  

Are smtp_generic_maps ment for this ? 

If so, is it okay, that my server sends an e-mail using specific envelope sender, and then something else in message being forwarded ? 

How else to deal with such problem ?
