This has been bothering me for a while, when using any kind of shader I've noticed the color is kinda bland in comparison to the actual color selected. As I'm working from reference art it's a bit of a bother when all the colors are somewhat off, despite being picked directly from the image. This was never a problem in 2.79b Cycles for me. Does not seem to be a result of lighting, as both Emission and Diffuse Shader to RGB produce these.
If you want the render to show exactly the color of it, you will need to directly connect an RGB node to the surface socket of output node to avoid any other calculation of final color.
And if my memory is correct, default view transform calculation behavior is changed in 2.80 Cycles. Did you accidentally change it away from default setting?
If some of these value is not as same as the screen shot, then it will cause the color picker not able to retrieve the original color.
I use blender to build some objects or scenes as reference to paintings as well. At the moment I am using a real color "pink cotton candy" and I had the same issue. My case was more about of trying diverse scalings and positions on the hdri ligthing, and changing here in and there material properties and subsurfaces interactions. I dont know about you but I use a white background while keeping HDRI effects, at first this wash out the contrast, but you just have to change the Ambient Occlusion distance on Render properties and it gets just fine.
Its slower than before, but on the other hand it kinda allows me to manipulate more the colors and contrasts in real time quicker, in the process I discovered a pallete more interesting taht the one I was intending so was worth it.