Is the dull looking solder on surface mounted components or through hole?
If it’s on through hole, it’s likely they were hand soldered so the dullness could be resin (cheap solders use resin, rather than more expensive fluxes in the core). If the board is clean of resin, it could be from soldering at the wrong temperature which will create a thin oxide film or sections of oxide on the surface. It won’t affect the quality of the join if it doesn’t penetrate too deep.
Chris: RoHS solder comes up much shinier than lead/tin because it doesn’t oxidise as quickly. Good quality lead free is tin, copper, silver - sometimes other metals. Cheap lead free is basically just tin which can oxidise quickly - it’s actually pretty hard to find solder that bad though (check dealextreme for some).