Paint.NET :: Edge Detect When Recolouring From Black And White

I have some black and white photos from my wedding and I want to add some colour to them. It looks like it will be too much effort to recolour the whole photo so I might settle for just colouring the waistcoats for a splash effect.

I have read lots of tutorials for Paint.NET and for GIMP [URL] Essentially I do the following:

Add a new layer
Use lasso to select the waistcoat
Use bucket fill with the purple colour
Select multiply for the layer.

This gives me the result I am looking for, but only in the region that I have selected.

Not surprising really, but something that I can't get right and that seems to be skipped over in all the tutorials is: how do I select the area correctly?

Unless I go down to a really high zoom and spend ages with the selection, I end up either not selecting everything or I end up with leakage into other parts of the clothes.

This tutorial [URL] for a paid for product seems to let you just roughly choose your selection then it uses some algorithm for edge detection to make the correct selection.

Is there anything in Paint.Net to choose the selection like this? I have tried the magic wand, but the since the picture is greyscale, it chooses random selections all over the image instead of just the waistcoat.