How to change the transparency color ?

The best way to get rid of transparency is by using the "Blend with Background" command in the "Image" menu and selecting a color of your choice and leaving Threshold on 0.

thanks but I want to keep transparency but changing its color, is there an option to change default rw's color for transparent background?

If you mean the color of the squares on the canvas, then no - there is no direct way to change them. The color are taken from Windows colors for dialog windows - if you change your Windows colors, the colors of the squares will changes as well.

I made it in Photofiltre this way:
-Disable the alpha transparency, using the menu Image > Mode > RGB
-Replace the background color by the desired transparent color using the menu Adjust > Replace color
How can I achieve this with RWPaint ?

Oh, you simply want to replace one color with another color. You got me confused as .png does not need a special color for transparency, it supports transparency natively.

There is no special command for this like in Photofiltre. The simplest way how to do something similar in RWPaint is with the Floodfill tool. Select it, set tolerance to 0, choose the new (transparency) color and then keep clicking on areas with the old (transparency) color.

First, notice that the image you open is named "Background" in the layer pane.
Create a new layer (first button on layer pane's toolbar) and drag it below the image layer.
floodfill the new layer with the desired color (that is, the color you wanted transparency to change into). Then merge the layers.
This should do what you wanted.

Oh, you simply want to reinstate one color with another color. You got me bewildered as .png does not need a special color for transparency, it supports intelligibility natively.

Couldn't find anything original to post Anonymous?