Making one color stand out can be done in different ways in Photoshop. The simplest way is with two Hue/Saturation Adjustment Layers on top of each other.

Step 1: First Hue/Saturation Adjustment Layer.

All shades, except red: set the saturation to -100

Step 2: Second Hue/Saturation Adjustment Layer.

Set all shades to -100 saturation in one go.

Step 3: Paint.

Now paint on the layer mask of the 2nd Saturation, with a black brush, which must retain its color.

For this effect if you use another photo, you’ll have to paint on the layer mask again.

Step 4: Finishing Touch.

Set the Levels to: