How to Preserve selected colors in a mono conversion

Initial selection

We want to preserve the red of the bus but not other red areas like the road markings or the coat or scarf on the pedestrians. So we’ll start by making a rough lasso selection around the bus — it doesn’t need to be very precise.

Color Range

We don’t want all the colors in the bus, such as the yellow mirrors or blue dashboard, so next go to Select > Color Range. Make sure the Select popup at the top of the dialog is set to Sampled Colors, and click in a midtone red.

Refine selection

Shift-click on more shades of red on the bus to add them; Opt-click to remove areas. The selection is white in the dialog’s preview; use a Selection Preview (bottom menu) to view the selection taking shape in the main window.

Invert selection

Click OK and you’ll see the selection in the usual way, as “marching ants” (although the color-based selection is subtler than this reflects). Now press Cmd+Shift+ I to invert the selection, so everything but the red is selected.

Add adjustment layer

Click the half-black/half-white button at the foot of Layers panel and add a Black and White adjustment layer to convert to mono. Adjust the sliders in the dialog to fine-tune by mixing the channel information differently.

Brush in refinements

Press B to switch to the Brush tool, and if needed reset your colors (see sidebar) to make white the foreground color. Zoom in, set the brush to a suitable size, and click the adjustment layer’s mask thumbnail to refine it.