Understanding the underlying objective truth about morality is absolutely fine. Morality can even be presently defined objectively as a psychological phenomena. The only thing you can't do is propose a scenario and ask whether or not it is objectively moral. To ask a question like that requires morality to objectively exist independent of subjective thought (which it does not). The only valid question you can ask about a proposed scenario is whether or not another person finds it moral or not, which is their subjective opinion.
Let me make sure I have you right. 'Blue' the color does not exist objectively. It can only be understood subjectively. Is this BLUE? If you can 'see' the blueness then you'd say yes. If not, you'd say no. The blueness is qualia. Actually, I'm not sure if that should be the blue is qualia felt as blueness?
Either way, the qualia blue is totally subjective. The subjective experience is dependent on objective neuroanatomy (ranging from photoreceptors through to the occipital lobe and onwards into that ocean of unknown we call consciousness).
Yet, because blue is initiated by a specific wavelength of light (say around 488nm) we talk of blue as if it, itself, were objectively real. As if blue was out there waiting to be discovered or 'sensed'. But, if I understand you correctly, blue is not 'real' (wavelengths are real) and so we should ultimately refer to wavelengths of light. And then understand that blue is the subjective experience of having one's photoreceptor stimulated by that wavelength and processes in the cortex.
Can we do the same for morals? Is there the equivalent of '488nm' for morality.... just waiting to be discovered?