There are a good 10+ shades of diamond white, greyer,whiter,bluer,reder,etc etc. Different paint manufacters same colours will also be different
You must also take into account nearly 20yrs of weathering and you will then see why white and red are two of the worst colours to match.
Matalics, pearls, and micas are easier most of the time as you always blend,
You can base and clear solid colours with blending but then the laquer you use must be CLEAR as most manufacters clear laquer isn't, also this will alter further colour changes with future weathering.
Paint manufacters will only match colours in daylight (natural light) Park under a halogen street light and it will look different again.
So a painters job is not just good application, It is the first thing you see and
winge about but is almost impossible to get perfect edge to edge colour match. I spent 20yrs in body and paint and it is a thankless task.