I consider the picture I am creating and select hardwood (mostly) that I think will suit that picture. I take into account the colour, the grain type, the flow of the grain, then mix and match them to produce a more pleasing effect .
Whenever I can, I use locally (to the Borders of Scotland) sourced wood but, as most types of wood in the UK are various shades of brown, it can be a little limiting. So, I will buy in more exotic woods, which are more colourful. As a result, my pictures will have red, orange, yellow and purple in them, all of which are perfectly natural.
When I work, I tend to choose the subject that feels 'right' at that moment, be it a chic cat, a lady writing a letter or cartoon characters.