How do i convert a Color3 value into a BrickColor?

What you're looking for is BrickColor.new(Color3.new(0, 0, 0)). As for BrickColor to Color3, BrickColor.new("White").Color.

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local brickcolor = BrickColor.new("Really red") --Create our new BrickColor
local color = brickcolor.Color --BrickColors have a Color property that returns their Color3

You could use color = BrickColor.new(Color3.new(0,0,0)) or color = Color3.new(0,0,0); brickcolor = BrickColor.new(color)

-- BrickColors RGB goes from 0 to 1, with decimal places 
-- color3 RGB goes from 0 to 255
local function ConvertBrickColorToColor3(brickColor)
    -- sampling rate of color3
    local colorSteps = 255
    local color3 = Color3.fromRGB(brickColor.r * colorSteps, brickColor.g * colorSteps, brickColor.b * colorSteps)
    return color3
end