Change color by expression - bar chart with multiple measures

[Rebecca]

Dear Community,

I want to change the colors of my grouped bar chart by expression. If I use the following function it is working for single bar charts (showing only the values of 1 week) but not for grouped ones (showing the values for 4 weeeks = 1 month).: 

if(colourcodestatus='OK', Green(),if(colourcodestatus='NOK', Red(), RGB(255,204,0)))

I have 5 different measures in my bar chart each of them shows how many NOK/OK parts I have (as percentage)
The parts can be made in 5 different stations. --> 5 measures
My x-axis is a timeline showing the month of the current year.

Following are the conditions: 
If there are 80 - 100% OK parts the color of the bar should be green. 
If there are less then 80% OK the color should be red as well as when there are more then 100% OK parts. 
PLUS if there is 1 NOK part in a month I want to see a yellow bar even if the percentage is actually 'OK'. 

Thanks in advance! 

Kind regards,
Rebecca