Select on colour range/blurry colours?

[denzjos]

To remove the colors you can use : Colours / Desaturate / Mono Mixer and move the 'Blue Channel Multipier' to the right.
To tune the result you can copy the layer (one or more) and set the layer mode to 'multiply' and then 'flatten' the layers to one layer
Then you can further beautify the result the way rich mentioned.