Q1.
Did you wonder why we decolourise the leaf in the beginning of this activity?
Answer
Because the leaf's own green colour would hide the result.
Iodine turns starch blue-black. On a dark green leaf, blue-black on green simply looks like "dark green" — you cannot tell whether the colour changed. Once the chlorophyll has been dissolved out by hot alcohol, the leaf is pale cream, and even a faint blue-black patch stands out clearly.
In the book's words: decolourisation of a leaf enables us to easily observe colour change and, thus, the presence of starch.
Tip: this also explains Activity 10.3. Only after decolourising can you see that the green patches of a variegated leaf turned blue-black while the non-green patches did not.