Found some values that give a good dynamic range from an fBm node, so tried replacing the Noise node with an fBm and plugged that into the Red input of a User_Defined node. Added a second and third fBm node to drive the Green and Blue, tweaked the x/y/z_index values in them to get a different pattern from the first. Now I have the complete range of colours available for the tiles:

Works in both FireFly and SuperFly, without those FireFly/Noise artefacts:

 

(note: the u-derived value is plugged into x_index, and the v-derived value into y-index and z-index. Using a constant produces the same problem as before)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident. And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU or something else.
*soon only to be available in two flavours -  DAZ and Renderosity