One word. colorifics.
You as the mixer of colours need to be aware of what tints go into the base mix and what exactly they do.
Give you an idea. A "cream" white for GM has 250g of extreme white. The next to go in is 38g of BLACK !
Now when this goes in it makes the most awe full mess and you think it will never work.
Next in is 3.8 g of Magenta. BINGO, it all comes together and you have your "cream" white colour.
The trick is to know what black will do and why Magenta brings it all together. Sometimes to get it right you may add red, yellow, blue etc..This is where the mistake is made. NEVER use a colour that is not part of the base formula !
In the case of the GT-r above it was a seven stage formula. We had seven options here to match the paint correctly.
Unfortunately with the modern day formula like for Renault, BMW, Nissan etc. the distance from the gun to panel and also the pressure will affect the outcome. Its all in the "lay" of the paint.
Matching from flaps and panels could be difficult if the guy does not have knowledge of the different variants of the same colour as used from the factory.
The OE factory will mix a ton of paint to paint for arguments sake 1500 cars on the line. When that runs out they mix a new batch.It is not always exactly the same so they will give it a code which is the same but with a different variance of the same colour.
Hope I described it such that it makes sense..
