colortest-256: 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/colortest-256 - from package: colortest

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

Error in other languages:
colortest-256: bevel nie gevind nie
colortest-256: comandă negăsită
colortest-256: comando não encontrado
colortest-256: commande introuvable
colortest-256: command not found
colortest-256: käsku ei ole
colortest-256: không tìm thấy lệnh
colortest-256: komanda nerasta
colortest-256: Kommando nicht gefunden.
colortest-256: kommandot finns inte
colortest-256: komut yok
colortest-256: nie znaleziono polecenia
colortest-256: níor aimsíodh an t-ordú
colortest-256: no se encontró la orden
colortest-256: no s'ha trobat l'ordre
colortest-256: opdracht niet gevonden
colortest-256: parancs nem található
colortest-256: perintah tidak ditemukan
colortest-256: príkaz nenájdený
colortest-256: příkaz nenalezen
colortest-256: команда не найдена
colortest-256: командата не е открита
colortest-256: 命令找不到
colortest-256: コマンドが見つかりません