How can I change this

I am trying to repaint A.F. Scrubb's BFC-2 Hawk Navy biplane textures and he has a green overlay in the extended bmp for the tail that no matter what I try in Photoshop I can't change to White which is what the tail color should be for USS Saratoga CV-3 in 1936. I wrote to "Scrubby" to see if he still has his PSD's which would make this so simple by just changing the layer but I have had no reply in a couple of days now by email. Here is a link to the bmp. I hope somebody here can help me out.

Here is the legal excerpt from his readme so I am sure I can share the bmp here to get help:

The project is released as freeware. You may modify it and repaint it. You may upload this file to another website as long as it is not for profit.
I kinda figured it out myself learning just one more thing about Photoshop. When I layer select the image there was a slider at the top that had a default of 40 in it. I slid this all the way to the left and it gave me a shaded white tail with the reg. no. and the aircraft designation of BFC-2. I saved it as a psd. Going to see how it looks on the aircraft in FSX because the background was also changed. In the meantime I can still use some feed back from ya'all here.
I tried and got this result pretty quick, just have to select all the green area, then make it black-white and up the brightness and a bit of contrast:

Of course you´ll always get a bit of quality loss when editing "baked" textures, so in my opinion best is to just ask the developer for a paintkit as you said.
Could it be that you also have an alpha/specular/bump or other map also that you have not taken into account?
no bump or spec files but the developer does not have a paint kit and told me to just use his texture. I figured this out as you can see above so all is well and we can close this solved. Thank you all for your help and input.