The regular material (for example V-Ray, Corona Renderer) can compute the Fresnel effect for dielectric materials like plastic based on the IOR of the material (n) – however metals have a more complicated Fresnel reflective curve that depends also on another parameter called extinction (k) coefficient:
η = n + ik.
n and k determine the two characteristics of the Fresnel curve for a material: the reflectivity at normal incidence and how quickly it fades to white at grazing angles.
The Texture map plugin can be used as a reflection color map in a material with the Fresnel option of the material turned off (this plugin does its own Fresnel calculations).
Presets – some presets for quick start (to enter other parameters (n, k) values it is necessary to select preset Custom).
If you need a bit different color (f. e. for various alloys), use a Color Adjustment parameters and adjust the Gamma, Hue, Saturation values. This way you still have the correct Fresnel curve and are just making color changes.
Gamma – specifies the gamma correction value.
Hue Shift – lets you change colors using a standard Hue spectrum. Range = -180…180.
Saturation – the intensity or purity of the colors. Lowering the Saturation value removes color, causing the color to tend toward grayscale, while raising it intensifies the color. Range = -100…100.
The site refractiveindex.info has the measured n and k values for many metals like Copper, Gold, Iron etc.