In the Scripts tab of your second HDA, at the bottom left there is an Event Handler field. Select On Created and it will produce a script module. Paste this code and save. When you create the colour_Two HDA, the colour_one HDA will be created and connected automatically.
(HDA naming should be kept consistent, e.g. colour_Two, colour_one <-- mind the capitalization!)
Python documentation for the methods I used can be found here.
hou.pwd() is shorthand.
kwargs is a dictionary, I'm accessing the "node" item.
# Get context
node = hou.pwd()
# Get the newly created node (self), use kwargs
self = kwargs['node']
# Create input node
c1_node = node.createNode("colour_one")
# Connect input node
self.setInput(0, c1_node)
# Layout
c1_node.moveToGoodPosition()