showrgb: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/showrgb - from package: xorg-x11-server-utils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/showrgb - from package: x11-xserver-utils

You may install the required package with command apt-get install {package}

Error in other languages:
showrgb: bevel nie gevind nie
showrgb: comandă negăsită
showrgb: comando não encontrado
showrgb: commande introuvable
showrgb: command not found
showrgb: käsku ei ole
showrgb: không tìm thấy lệnh
showrgb: komanda nerasta
showrgb: Kommando nicht gefunden.
showrgb: kommandot finns inte
showrgb: komut yok
showrgb: nie znaleziono polecenia
showrgb: níor aimsíodh an t-ordú
showrgb: no se encontró la orden
showrgb: no s'ha trobat l'ordre
showrgb: opdracht niet gevonden
showrgb: parancs nem található
showrgb: perintah tidak ditemukan
showrgb: príkaz nenájdený
showrgb: příkaz nenalezen
showrgb: команда не найдена
showrgb: командата не е открита
showrgb: 命令找不到
showrgb: コマンドが見つかりません