xcolorsel: command not found

Have You tried accessing the binary with the full path?

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/xcolorsel - from package: xcolorsel

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