Colour is a complicated thing. It should be quite simple, but it isnt. Ink printed onto paper can be compared one with another, relatively easily (under standardised lighting).
Monitors are trickier... but eventually they can be calibrated, colour 'spaces' unified and so on to attempt to get technically identical colours rendered loking like they are the same colour...... but nothing really prepares you for just how different they can look once you've got everything technically perfect!
The juxta position of colour and..... the brain!!
R Beau Lotto is a lecturer in Neuro Science at University College London (UCL). All images supplied by R Beau Lotto on this webpage, there is the best demonstration we've ever seen of why we are right and the customer is wrong hehehe.