I've not got Phopto Designer here, but most photo editing programs have a "flood fill" option by that or some other name.You pick the color you want to use and then click anywhere in an area of the color you want to repace. There are usually some options:
Sensitivity -- change only the pixels of exactly the chosen color or replace a range of near matching colors. This is useful if your cream wall is not evenly illuminated.
Local or global -- global change all the pixels of the chosen color no mater where they are. A global replace might add blue spots on a face that matched your cream target color.
An alternative is to first draw a 'mask' covering all the areas you don't want to change.
Check out Color Editor., Masks and Layers in the installed pdf manual - this should be available from the Windows Start menu for the program.
With these you can selectively change colours and avoid, as Carl says, using a global colour change which can change similar colours that you may not want to be changed.