The format of the MaterialColors property is an array of colors. Each group of 3 bytes represents a Color3 corresponding to a particular Material enum item. The 3 bytes in each group correspond to the R, G, and B components of the Color3.
The array corresponds to a predetermined, constant list of materials. The first two groups appear to always be zeroed. It is possible these are reserved for the Air and Water materials.
RR GG BB
[00 01 02] Unknown (Air?)
[03 04 05] Unknown (Water?)
[06 07 08] Grass
[09 0A 0B] Slate
[0C 0D 0E] Concrete
[0F 10 11] Brick
[12 13 14] Sand
[15 16 17] WoodPlanks
[18 19 1A] Rock
[1B 1C 1D] Glacier
[1E 1F 20] Snow
[21 22 23] Sandstone
[24 25 26] Mud
[27 28 29] Basalt
[2A 2B 2C] Ground
[2D 2E 2F] CrackedLava
[30 31 32] Asphalt
[33 34 35] Cobblestone
[36 37 38] Ice
[39 3A 3B] LeafyGrass
[3C 3D 3E] Salt
[3F 40 41] Limestone
[42 43 44] Pavement