Chromogen enhancers are just color modification used to vary the intensity of the color appearance. 3,3’-diaminobenzidine (DAB) enhancers contain compounds that continue the DAB reaction or darken/change the color. An enhancer to the color can be heavy metals, such as copper, silver, nickel, gold, or cobalt. The most common metal is nickel, which gives a blue-black color to the DAB. For electron microscopy IHC procedures, gold is used.
In any case, when using an enhancer for the first time, a control slide using the DAB without enhancers should be stained prior to adding the enhancer. The presence of the original unenhanced reaction validates that the enhancer is not turning a negative stain result into a positive stain result.