I have three plates that are all the same. The one in this pic is in my 75G and the other two are in my 29G. This one looks as you say, a blueish gray, in the posted pic and in person. This one is under T5's with daylight and actinic lighting.
The two in my 29 look more brownish. They are under an one T5 and I don't think it is a 50/50 and may just be more of a natural light.
My best guess is in a tank without any or little actinc it will be brown.
You could pick from any of the three, but I am confident they are all the same color.