In terms of countering the opponent, let's look at the order:
- Your opponent goes for some Gateways.
- You create a strong bio ball.
- Your opponent counters them with Colossi.
- You spend less on your now weaker bio ball, and spend that money to get vikings.
- Your opponents Colossi is now weaker, so he could spend money to counter your vikings.
So, unless your opponents goes for an odd play style, you can conclude less Colossi are made.
Also, you can easily suspect Colossi when creating a Bio Ball; and Vikings when going Colossi.
Just try to scout Robotics, if not, scout for Twilight Council (--> HT/DT) or a Stargate (--> Carrier).
Scouting is key to make strategical decisions supported by risk analysis, in military terms:
There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.
Scouting and map control helps you increase the amount of known knowns. :-)