Create a CMYK composite image (pdf) from individual color separations

Yes, this is what I’m trying to do. I have succeeded in rasterizing the initial grayscale pdfs (exporting as png), colorizing (with cyan, magenta, yellow), applying transparency (value of 1), and then adding them together to create a new pdf with the blend mode ‘darken’ (which is the only blend mode that gets me close to the final image) but the image is not quite right - the color is a bit off and not saturated enough…Is there a difference if I would make a new ‘multi-channel’ pdf? How would I create these new ‘channels’ from the initial grayscale pdfs?