Material colors

[CCS]

Here's a problem I run into fairly regularly.

Client comes wanting a building exterior rendering. You model it up easy enough. Set up sun, GI, etc. and render it with an override material that gives a satisifactory blend of light:shadow, warm:cold, daylight:sunlight, bluerange, etc...

Now time to start applying real materials. Client says they want a particular color (RGB). I plug that in and hit render and it's not even close. I understand the reasons why, but does anyone have a solution other than trial and error? Something I can do to adjust each color (by H, S or V maybe) ahead of time?

I'll post images later to illustrate.

[rhino]

Well this is why programs like finalrender have a diffuse level, diffuse is different to colour, it's the amount of light absorbed by an object. (someone technical can jump in here) Ideally Vray material would have a colour and diffuse slot this would allow you to make your swatches at the colour RGB equivalent and then control the actual absorption values of the material with the diffuse value or slot . i.e. the more shiny a material the more light that is reflected back to the eye/less absorption, while a matte material would be absorbing more light. The two materials could be exactly the same "colour" well that's not exactly how it works but close enough for me to understand.
So what to do? well you either adjust the black level of your swatch until it renders correctly which makes it a bit of a pain to remember exactly what that colour was meant to look like or what I do a lot of the time is place either a checker map or a script that allows you to put a colour swatch in a map slot, then control the black amount by applying an output map in the diffuse slot and reducing the RGB level of the swatch. Once you have the colour rendering correctly apply that same output amount to every other material of the same type.
Or solution 2 is use standard materials, set it to Multi-Layer and then simply adjust the diffuse value.
Can I say once again Vray material should have a diffuse value as it would make architectural work and colour matching way way easier.