As a Roblox developer, it is currently impossible to change the color of the corroded metal material, and other materials with baked in colors.
I would want to do this to use materials like the corroded metal texture for new and interesting purposes; e.g. as copper oxidation (which is green), perhaps as some kind of mold or fungus, etc. Currently materials with intense coloration are way too restrictive to ever be useful to me.
Further, Roblox is currently developing new materials that have so far proven likely to contain some amount of coloration in the diffuse texture.
For example, this is cobblestone on a grey part in the build provided in the thread linked above. It has a beige color baked into it to allow it to match terrain cobblestone more closely.
Perhaps in part due to community response, Roblox has assured that most instances of coloration will be removed in these textures, but if the intention is to match part textures to terrain textures, this is a bad idea, and will make matching sometimes impossible since terrain textures can contain more than one color, and arguably should contain more than one color because that is more visually interesting.
You can still see that this part is not terrain, it is too green where it should be yellow.
Inversely, if coloration is left on materials, since terrain can be recolored but materials cannot be recolored, this would also break the ability to match terrain materials to part materials, (in addition to destroying material flexibility).
You can see here if I make terrain’s leafy grass blue, because of the colors baked into the part material, I can no longer match the color.
To solve this problem, a new property to BaseParts should be introduced to change the hue of the material’s diffuse via rotation, similar to the way terrain materials can have their hue rotated (e.g. if you change the color of the rock texture on terrain, the secondary colors in its material also shift).
This would enable part materials to more closely match terrain materials, enable more interesting materials by virtue of being able to safely contain more than one color, and increase material usage possibilities for developers by not locking them to certain colors.
E.g. to match a part to recolored terrain, the developer would set the part color to pure white, and then hue shift the part’s albedo to match the terrain material hue.
This will automatically afford developers a way to have materials that take advantage of both colors for increased material reusability (important for memory optimization). If Roblox offers the ability for developers to take advantage of tint-masks in custom materials (i.e. ignoring part color for corroded metal, or the grout between bricks).