There is no problem for me now to change color for pixels of the same color: I click on "Select by Color Tool"(Ctrl+O). The pixels of the same color are selected now. Then I press Ctrl+,. The selected pixels are white now (the white color is taken from a foreground color).
The problem is the transparency. The green color I want to change to the white one has transparency. Once I use my method, the transparency disappears so all my-green-color with any transparency becomes white and without transparency.
How do I keep the transparency by using my method?
My picture is in .png format.