OpenOS. Colors Table.

local conv = require("conv")
local component = require("component")

local rr={"00","33","66","99","CC","FF"}
local gg={"00","24","49","6D","92","B6","DB","FF"}
local bb={"00","40","80","C0","FF"}
local bw={"0F","1E","2D","3C","4B","5A","69","78","87","96","A5","B4","C3","D2","E1","F0"}

local x,y=1,1
local gpu = component.gpu
local w,h = gpu.getResolution()

gpu.fill(1,1,w,h," ")
for _,r in pairs(rr) do
  for _,g in pairs(gg) do
    for _,b in pairs(bb) do
      local rgb=conv.getColor(conv.getThreeColor(r..g..b))
      gpu.setForeground(conv.invertColor(rgb))
      gpu.setBackground(rgb)
      gpu.set(x,y,r..g..b)
      x=x+7; if x>=w-6 then x=1;y=y+1 end
    end
  end
end
x=1;y=y+1
for _,i in pairs(bw) do
  local rgb=conv.getColor(conv.getThreeColor(i..i..i))
  gpu.setForeground(conv.invertColor(rgb))
  gpu.setBackground(rgb)
  gpu.set(x,y,i..i..i)
  x=x+7; if x>=w-6 then x=1;y=y+1 end
end