
It really was a doodle so it won't be a reliable account but I split the image twice in RGB and CMYK, discarded all but the red, cyan and magenta. Then I tweaked the levels of each to get the best contrast of tree against sky before combining the three channels in RGB: red to red, cyan to green, and magenta to blue. Unsharp mask was used to create contrast and darken the greys. Then I copied the original colour image as a new top layer, pushed the saturation and lowered the transparency just to give the merest whiff of colour - I'll still call it B&W.