How to change the color of all parts in workspace if they have a certain material?

You have to do something first before you even consider changing BrickColor and Material.

I highly suggest that you check to see if the child is a BasePart using Instance:IsA(). This is because if the for loop encounters a userdata that isn't a descendant of the BasePart class (which means that the child isn't either a Part, MeshPart or UnionOperation), it will error out because BrickColor and Material will not be valid properties of that child. This can easily happen if you have a few models assorted throughout the workspace. Models are not descendants of the BasePart class, and therefore they do not have the BrickColor or Material properties.

For assigning the Material property to a value, strings have been deprecated. Enums are now heavily favored for changing the Material of a BasePart. A full list of material Enums can be found here.

Here is an example of what I just explained to you:

for _, inst in pairs(workspace:GetChildren()) do
    if inst:IsA("BasePart") then
        if inst.Material == Enum.Material.Grass then
            inst.BrickColor = BrickColor.new("Institutional white")
            inst.Material = Enum.Material.Sand
        end
    end
end

Something you're doing wrong:

if Ground.Material == ("Grass") then

What you're doing wrong here is using a string instead of an Enum property.

if Ground.Material == Enum.Material.Grass then

Also, it should be a lot better just to use a loop.

for i, v in pairs(Ground) do

Here's the improved script:

Ground = game.Workspace:GetDescendants()
wait(0.5)
for i, v in pairs(Ground) do
    if v:IsA("BasePart") then
        if v.Material == Enum.Material.Grass then
            v.Material = Enum.Material.Sand
            v.BrickColor = BrickColor.new("Institutional white")
        end
    end
end

I hope this helped!

local children = game.Workspace:GetChildren()

for i = 1, #children, 1 do
if children[i].Material == "Grass" then
children[i].Material = "Sand"

end
end