Reading the colour
One drop of iodine, one of two answers. There is no third possibility and no judgement required.
Blue, or dark purple
When iodine solution is added to the leaf, if it turns blue or dark purple, it can be concluded that starch has been produced — and therefore that photosynthesis took place.
No colour change
If the iodine stays its own brown-orange colour, there is no starch in that leaf, and photosynthesis did not take place there.
You already know this test
Grade 10 used exactly the same reagent on food in a test tube: add a drop of iodine solution, and a blue-black colour appears if starch is present. Nothing about the chemistry has changed. What is new is only the preparation — a leaf has to be killed, decolourised and softened before it will behave like a sample in a test tube.
A leaf can give different answers in different places
The result is not one colour for the whole leaf. Part of a leaf can turn dark while the rest of it does not, and that is not a failed experiment — it is the entire method of the next three skills. Where the colour changes tells you where photosynthesis happened.
Blue-black means starch, and starch means photosynthesis
Iodine turning blue or dark purple shows starch is present, so photosynthesis took place in that part of the leaf. No colour change means no starch, and no photosynthesis.