Gryffindor and Slytherin colors

[bercygirl2]

No: HPFGUIDX 161775

Somthing has been nagging at the back of my brain about these two 
houses' colors. Gryffindor's colors are gold and red, Slytherin's are 
silver and green.
But in HBP we are told that the locket and ring that were Salazar 
Slytherin's are gold, not silver, as we'd expect his heirlooms to be. 
And in CoS, Gryffindor's sword is silver.
Does anyone have any theories about this?