Corona SDK use just RGB colors. This function convert HEX to RGB.

hex2rgb.lua

-- path for this file: src/utils/

function hex2rgb (hex)
    local hex = hex:gsub("#","")
    if hex:len() == 3 then
      return (tonumber("0x"..hex:sub(1,1))*17)/255, (tonumber("0x"..hex:sub(2,2))*17)/255, (tonumber("0x"..hex:sub(3,3))*17)/255
    else
      return tonumber("0x"..hex:sub(1,2))/255, tonumber("0x"..hex:sub(3,4))/255, tonumber("0x"..hex:sub(5,6))/255
    end
end

main.lua

-- main.lua is required in root folder on projects with Corona SDK

require('src.utils.hex2rgb')

display.setDefault("background",hex2rgb('#1b1a1a'))