ppmcolors: 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/ppmcolors - from package: netpbm-progs

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

On Debian possible default full paths are:

/usr/bin/ppmcolors - from package: netpbm

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

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