Is it possible to leave (isolate) only one colour in image? Currently I'm interested in green:005d00
Well you can do it using ImageMagick via the command line:
convert original.png -matte ( +clone -fuzz 1 -transparent #005d00 ) -compose DstOut -composite isolated.png
The -fuzz command can take a percentage variance from the colour, if it's specific, drop the fuzz.
The () brackets need escaping in bash shell \( \), etc.