I have this problem all the time. Mainly with one customer who i think has their monitor so bright you could {fill in the blank} I will spend hours designing a site only to be told at the last minute that the colours look wrong on their monitor. I do try an explain that depending on the monitor it will look different but it is often difficult to get through and easier to spend the time making it look right on their monitor rather than everyone elses. The customer is always right! There was one occasion where i did this only to be called the next day and told that they didnt like the way it looked on a colegues monitor 'could i change it'