Using Color to Alpha

Choose the color picker tool from the toolbox

Place it in your image over the color you plan to make transparent

Click the left mouse button, the color picker confrmation box appears

If the color is correct, click Close

Double-click on the new Foreground color in the lower-left of the GIMP toolbox-- it should have changed to the color you just picked.

When the color picker dialog appears, the new foreground color values will be present. Highlight the HTML_Notation value, then click Ctrl+c to copy that to your clipboard.

Close the color picker dialog. Now you are ready to do the color to alpha.

Select Layer>Transparency>Color to Alpha from the menu.

The color bar between From: and To Alpha will appear white (unless you have used the tool since openning the GIMP, in which case it will be the last color you used in this particular dialog.)

Click on that color bar to open the "Color to Alpha color picker"

Highlight the HTML_Notation value and paste in Ctrl+v the value you sent to the clipboard earlier.

Click OK to set and close the color picker.

The new color should appear in the "Color Picker" dialog.

Click OK