When I draw a rectangle shape in Photoshop it has colour properties.
But when I draw a line it has no colour properties and just uses currently selected colour.
I want to change the colours of lines I have already drawn. How do I do this?
As commented under question, pressing A solves the problem. The thing is, when you've selected a tool like Move tool (V), you're not in control of shape attributes, but you're more in control of object's positions and alignments.
When you select Direction Selection Tool or Path Selection Tool (A), it selects the part(s) of object and hence you get the options to alter them.