rainbows: 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 Debian possible default full paths are:

/usr/bin/rainbows - from package: rainbows

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

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