1. Open the file in Photoshop
2. Go to Image->Mode and make sure the colour is on RGB not Indexed Color
3. Go to Layer->New to add a background layer. Ensure the Color drop down is set to "None", Mode is set to "Normal". Change the Opacity to 0%. Click OK.
4. Right click the image layer (not the transparent layer you just created) and select "Convert to Smart Object"
5. Double click the image layer (You'll get a warning, click OK)
6. Click Select->Color Range, then move the dropper over the image to the color you want to be transparent. Click OK. The outline of that color should be selected.
7. Goto Edit->Clear
8. Save the image and you are finished.