I can't come to terms with Colorspace

[restlessdan]

I have been making some watch faces and experimenting with different looks and feels and I recently put into my current project settings to change the colours of the hands and watch dial.

so I have been using inkscape to make my hand and dials for my watch when they are exported into the studio they look nice with perfect straight lines and curves but to change the colours I know I need to make them grayscale but when I use the commands:

convert img_in.png -colorspace gray img_out.png

or I have tried 

convert img_in.png +profile "*" img_out.png

and after I do this I always end up with the same results jagged edges, does anyone know how I combat this so that they keep the same smoothness that they are exported with.

many thanks for any replies.

[JonFitbit]

Is your source image 16bit png? Can you provide some of the images you're using?

[restlessdan]

hi sorry for my late reply I was away from my computer this weekend, after double checking its a 32-bit file. 

had to zoom in on that picture otherwise you can't tell how jagged it is but definitely worse to the eye. is my problem that its not 16 bit?