Hi Laura,
use welch-- where you may use "which" in English, so which colour as opposed to what colour.
In detail:
1. ‘What do you see in the shop window?’
Was/welche sehen Sie im Ladenfenster?
richtig.
If the answer were "Jacken und Hosen" you may ask
Welche Hose gefällt dir am besten?'
2. Was liest der Junge?
richtig
Welches Buch liest der Junge would imply that you have a few choices (das dicke, das neue, das gelbe, das grüne oder ...?)
3. Was macht die Frau?
richtig
and no variation possible