ImageMagic colors not as bright as Photoshop when converting to CMYK PDF

My prospective printer demands a CMYK PDF. I have 40 PNGs to prepare (repeatedly). I convert an RGB PNG to a CMYK PDF with ImageMagick like this: convert myRGB.png -colorspace cmyk cmyk.pdf

The colors are not as bright in this pdf as they are if the same operation is done in Photoshop. I do not own Photoshop (a friend supplied the example). There is a web site that produces the same brighter result, but only one file at time. I need an open source solution like ImageMagick.

I have tried using several ICC profiles that I have downloaded from Adobe with the convert command like this: convert myRGB.png -colorspace cmyk -profile someAdobe.icc cmyk.pdf

But the results are identical. Can someone explain how to get brighter colors and also why the convert with -profile does not have any noticeable effect?

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