colortest-16b: 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-16b - from package: colortest

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

Error in other languages:
colortest-16b: bevel nie gevind nie
colortest-16b: comandă negăsită
colortest-16b: comando não encontrado
colortest-16b: commande introuvable
colortest-16b: command not found
colortest-16b: käsku ei ole
colortest-16b: không tìm thấy lệnh
colortest-16b: komanda nerasta
colortest-16b: Kommando nicht gefunden.
colortest-16b: kommandot finns inte
colortest-16b: komut yok
colortest-16b: nie znaleziono polecenia
colortest-16b: níor aimsíodh an t-ordú
colortest-16b: no se encontró la orden
colortest-16b: no s'ha trobat l'ordre
colortest-16b: opdracht niet gevonden
colortest-16b: parancs nem található
colortest-16b: perintah tidak ditemukan
colortest-16b: príkaz nenájdený
colortest-16b: příkaz nenalezen
colortest-16b: команда не найдена
colortest-16b: командата не е открита
colortest-16b: 命令找不到
colortest-16b: コマンドが見つかりません