We report here the preparation of co-assembled microcrystals by employing an easy, reproducible and cost-effective technique, namely slow diffusion. 2H-Benzo[d][1,2,3]triazole and benzo[c][1,2,5]thiadiazole were chosen as host and guest skeletons, respectively. Structural similarities allowed the correct co-assembly of the two structures. The co-assemblies were studied by different techniques that included Raman spectroscopy and X-ray diffraction, amongst others. The waveguiding properties and the emission colour of the doped organic microcrystals were also investigated. It was found that changes in the molar ratio of the different doping agents could tune the light emission. Fluorescence microscopy images of the co-assembled microcrystals revealed light colour changes from green to whitish, up to CIE coordinates of (0.370, 0.385). These tunable colour-active materials could be useful in the fields of optoelectronics or lab-on-a-chip for integrated optical circuits at micro-/nanoscale.