Why is my for loop only printing 1 part out of the multiple in the folder?

It's because the while true do stops the whole entire for loop. It just focuses on what's inside the while loop and not outside it

What I recommend you doing is making a seperate Script for the changing color. Then put it to someone then name it to anything, AND BE SURE TO DISABLE IT. For now, let's name it "ColorChange"

-- Color Change Script:

while true do
            wait(.2)
            script.Parent.Color =   Color3.fromRGB(math.random(1,255),math.random(1,255),math.random(1,255))
end

Then in your main script

-- Main Script:

local script = script.Parent
local folder = script:GetChildren()

local colorChangeScript = wherever.you.put.it.ColorChange

for _, v in pairs(folder) do
    if v:IsA("MeshPart") then
        print("hi")

        local colorChangeScriptClone = colorChangeScript:Clone()
        colorChangeScriptClone.Parent = v
        colorChangeScriptClone.Disabled = false
    end
end

If it helped, be sure to accept it as an answer :D