Engine oils of the same performance level / specifications are compatible and hence can be mixed / used for top ups.

No.The type of oil is specified based on the design of the vehicle and it is normally given in the OEM guidelines(OEM)

Colour does not affect the performance of the oil. Different  colour dyes are added to differentiate brands. Red colour masks oil colour which otherwise looks dark and blackish due to impurities.

Colour does not affect the performance of the engine oil. Different components in the engine oil can make the colour of the oil different.