Well... maybe...
In theory, a transistor rated for HF (high frequency) signals should be used rather than a general purpose one. A composite video signal has frequencies between a few tens of Hz through to 6MHz.
The larger the value of capacitor, the greater the frequency range it will pass. However, electrolytic capacitors become slightly inductive at higher frequencies. So 100nF ceramic types are sometimes wired in parallel with them. Also larger values of capacitor allow more current to flow.
The value of the resistor should really be determined in order for the circuit to have a 75 ohm output impedance. 100 ohms is not too bad, 120 ohms is a bit further away...
But I'm not absolutely convinced making any changes would make much difference. As most of these 'buffers' designs were put together by experimentation.
It could be that your monitor does not like the non-standard video signal from the ZX Spectrum, or it could be that parts of the signal it is receiving are marginal, hence dropping from colour to monochrome.
Mark