In this topic, we can gather tutorials which show how to isolate a single colour and leave the rest of the video black and white. (Most people know this as the "Sin City" film effect.) I know that there are different ways to create this effect. Here are two tutorials, using the Color Corrector in SV:
Sin City Effect - Sony Vegas Tutorial >>> Sony Vegas 8; Using Color Corrector (Secondary)
Sony Vegas - Color Splash >>> Sony Vegas 9; Using Color Corrector (Secondary)
If you have different suggestions or found other tutorials to create this effect, feel free to post them here!^^ I will add all suggestions and links into the first post, so we will have a list of different ways to create this effect then.
I'm not sure if this is exactly what you were looking for Alyvia, so let me know if you need more advices^^