How does a camera pixel accumulate charge (colour)?
The charge is accumulated on all the capacitors connected to the cathode of the photodiode. This means:


The source-bulk junction capacitance and gate-source overlap capacitance of \$M_{rst}\$
The gate capacitance of \$M_{sf}\$
The photodiode junction capacitance
Any stray parasitic capacitances (eg. trace capacitance of the metal layers)


You usually want to have a small total capacitance to have the best signal-to-noise ratio for measuring the number of photogenerated charges stored on those capacitors.