Identify the colour of compound 'X' in the sequence of the reaction.

The given reaction sequence represents the synthesis and acid-base behaviour of phenolphthalein.

When phenol reacts with phthalic anhydride in the presence of concentrated $H_2SO_4$, phenolphthalein is formed. In acidic or neutral medium, phenolphthalein exists in a lactone form which is colourless.

Upon addition of a moderate amount of alkali (NaOH), the lactone ring opens and a proton is removed, forming a dianion with an extended conjugated quinonoid structure. This extended conjugation is responsible for the characteristic pink colour of phenolphthalein in basic medium.

When excess NaOH is added (strongly alkaline medium), a hydroxide ion attacks the central $sp^2$ hybridized carbon atom of the quinonoid structure. This converts the central carbon to an $sp^3$ hybridized state, forming a carbinol structure (compound X).

Because the central carbon is now $sp^3$ hybridized, the extended conjugation is broken. As a result, the compound loses its colour and becomes colourless.

Therefore, compound X is colourless.

Answer: Colourless