This technique can make some eye catching images foy our website. I will let the videos explain themeselves along with my notes.

How To Colorize Black and White Photo in Paint.NET (Windows) - YouTube

• set background/bottom layer as a black and white image
• create a new layer (should go above), call it colourize
• edit the colourize layer properties, change 'Blending Mode' to overlay (or your preference)
• make sure the colourize layer is selected
• draw/fill (with paint brush) the area that you want to colourize
• ? - edit the colourize layer properties, change opacity to 150

Paint.Net - How to add color to ONE item in a black and white image - YouTube

• set background/bottom layer as a black and white image
• create a new layer (should go above), call it colourize
• make sure the colourize layer is selected
• draw/fill (with paint brush) the area that you want to colourize
• edit the colourize layer properties: Blending Mode' to overlay (or your preference), change opacity to 150 (or your preference)
• Goto Effects->Gaussian Blur (25 is a good option); this removes jagged edges

NB: if you setup the new layer befor doing the drawing/filling you can actually see the effect happen in real time

My Method

• add black and white image as bottom layer
• add a new layer called colourize
• set colourise layer properties: Blend Mode --> Overlay, Opacity --> 200
• select colourise layer
• set your primary colour to that you want to hightlight in
• using the draw tools, pencil/paint brush/magic lasso etc.. colour in the area

Cut out area method - this is a litte more tricky

• Colourize Layer
• duplicate the bottom layer
• cut out the objects you want to keep so only they are the only things left
• then fill them in with your colour
• convert this image into an overlay layer
• ? - Background Layer / Black and White image