how to change faceplate header colour in 800xA aspectserver?
Graphics colors are defined for each workplace using Workplace Color aspects. Look in the workplace objects in the Workplace Structure.
Unless you have a strong reason for changing faceplate colors, I would recommend you dont. You'll need to change it in multiple places if you have several workplaces ( eg Engineering, Operator, etc) and you may find that its difficult to find a color that does not create problems or clashes with other graphic elements.