I have opened a colour image and converted it to B/W(Enhance>Convert to Black and White).
To add colour to the black and white image and make the car blue and the driver’s helmet red. Layer>New layer name it colour. Change the layer blending mode to Color in the drop-down menu.
Select the brush tool from the tool box and set the background colour to blue or any other color you wish.
To change the colour of the brush click on the squares at the bottom of the toolbox, the default is black and white. The foreground colour is ontop the background colour underneath. Clicking on the foreground color opens a color picker window.
Using the brush tool paint in the blue colour on the car body, I used a brush size 100px, opacity at 100%.
It doesn’t matter if you go over the edges as the next step is to use the eraser to rub out those bits.
The final stage with this image was to set up another new layer and repeat the above process to paint the driver’s helmet red.