Every wall switch plate and electrical device manufacturer produces a slightly different shade of color, such as "ivory" or "brown", so items generally will coordinate rather than match exactly.
Perceived color is also affected by finish of items as well, i.e. gloss vs. satin.
When attempting to match a currently installed plastic electrical device or wallplate, please keep in mind that your old items likely have darkened over time making an exact match impossible.
Please see Ivory and Almond Comparison for additional information.
Check out and compare device finish with cover plate finishes here.