I was trying to make myself a simple tool to display an integer using resistor colours, but some of the colours I was choosing display completely wrongly.
In the example here, I used the Colour Picker to choose (143, 72, 19), which is a fairly rich brown colour, shown correctly in the two little squares in the bottom-left of the picker, but when I then use the Custom Preview to apply this to a plane surface, the colour comes out as a much lighter orange colour.
I find these automatic type conversions that appear to ‘just work’, but may actually do something unexpected, to be so confusing! Colour → Material worked for some colours and failed for others, but only in wire-frame mode!
But yes, fixing Diffuse and Specular to Black and using only Emission for the colour give the right answer in all display modes.