Recent progress has boosted the resolving power of optical microscopies to spatial dimensions well below the diffraction limit. Yet, axial super-resolution and axial single-molecule localisation typically require more complicated implementations than their lateral counterparts. Also, there is a lack of suitable calibration tools that would permit a facile metrology along the microscope’s optical axis. Axial test samples are also needed for quantifying nanometric drift, or axial fluorophore mobility, or again for quantifying the sub-wavelength light confinement in near-field microscopies. In our work work, submitted to Opt Commun and now available as a preprint on arXiv, we propose a simple solution for axial metrology by translating an axial metrology challenge into a spectral imaging and mixing problem, by aid of a multi-layered single-excitation, dual-emission test slide, in which axial distance is colour-encoded.