Changing the colour of a wall with Cathedral Ceiling?

[Bigfellaphil]

Hello! hope someone can help....

My design has an open plan Kitchen with a Dining area separated with an invisible wall the cathedral Ceiling straddles both rooms. My problem is, i need to colour one halve of the wall one colour and the other half another colour but when i attempt to do this, the bottom portion of the wall changes but the top portion of the Cathedral wall stays original colour, when i attempt to manually change it both top halves change colour. I'm sure there must be a way to have the whole half of one Cathedral wall one colour and the other half another colour??

Thanks in advance,

Phil