I’m trying to render jewelry in rose gold. Just wondering if anyone knows a way or a color hex to achieve a look similar to the below image. Thank you for your time!
Basing creative decisions on scientific measurements
It is only possible to base your creative process and guesses on some scientific measurements while the results are never 'physically accurate' or 'correct' in CG. They should not be. Nobody actually needs this accuracy. It is OK to aim for a nice image that would be aesthetically pleasing and not to pursue physical accuracy that is most often impossible to achieve. Think of it this way - do you know the wavelengths of the primary colors of the monitor you are going to view the images on? I didn't think so. So let's not pretend anything about the images we create is even close to physically accurate.
If you wanted to use the data at https://refractiveindex.info to approximate the reflectance for the shader, you would need to know what the primary RGB color wavelengths you need are. I don't know what they should be, so let's just think that this random calculator that I just Googled gives us somewhat correct goals of what the ideal primary wavelengths for sRGB should be. It says red is around 610nm, green is around 550nm and blue is somewhere around 460nm. We can get the graphs for similar wavelengths at refractiveindex.info if we input the lengths here(in micrometers instead of nanometers though):
Only then you can take three different graphs like this one for all the primary wavelengths and replicate them using RGB Curves.
You should also ignore green and red lines representing different polarization, because in CG, we do not care about polarization at all. So only green lines for wavelengths representing red, green and blue colors.
But wait, what now? What should control our RGB Curves node? - you may ask. And that would be a wonderful question. Whatever it is - it is clear that it certainly is not some UV coordinates! We have the graphs from refractiveindex.info for Reflectance and Angle of Incidence in degrees that go from 0° to 90° linearly. Nothing in Blender actually gives us that.
Layer Weight and Fresnel nodes do not give us brightness depending on the angle of incidence from 0° to 90° linearly.
We have to actually make it by taking the arc-sine of the dot product of incoming and normal vectors and divide it by half of pi and then inverting it by subtracting it from 1:
(Sorry for added confusion - I use Bump Map node to get the normal vector because I want a way to plug normal or bump maps to this whole thing later.) Obviously it is better to have this as a node group.
So now let's just assume there is some micro-roughness effect visible because of the way the gold is polished(no reason other than that it looks nice on metals) and add roughness variation depending on the facing angle of the surface and we have something similar to what gold might look. Obviously it needs to be lit nicely as well and have some environment to reflect:
To make rose gold you would mix that with something like 25% of copper.
Is this process worth it?
I am not so sure it is for every situation. It is time consuming and it is also questionable if the extra effort is better noticeable than just working with some good reference photos. If you googled 'Gold' and compared it to my render you would find all kinds of different colors. That is normal and to be expected. They are all lit differently in different environments therefore reflecting different things, taken with different cameras and also all post processed differently. I think it makes a lot of sense to better invest more time into presentation, the models and especially the lighting and composition of the renders to make an impact on the viewer. These things can ruin all the effort of inputting loads of measured numerical values if not done correctly. It is not about physics, it is about visual communication. And this is just roughly based on some measurements. The rendering process is not physically accurate by any measure. I do however think it can be a strong base on which one could build an image if one has the time to play with this.