They also lighten as they age in the sun etc. How old is the old paint?bought some 'touch up paint' for my landy, which is 'orkney grey', a medium/neutral grey colour
the paint is a dark, slate colour.. i know paints lighten as they dry.. but i've done a test (not on the car) and its nowhere even close
Have you tried mixing it with a little white paint?
Do you have the paint code for the car?
Could you take a piece of the car with you to somewhere like Halfrauds where they will paint match to what you have rather than go by the old code?
Or maybe you could paint a piece of summat with a lightened mix of the grey you got and the white and get them to mix a rattle can.
Best of luck.