First of all you need the console / autoexec

All you need is binding.

Put this into your console / autoexec
alias +changec "+attack; (Color on shoot)"
alias -changec "-attack; (Default Color if not shooting)"
bind "mouse1" "+changec"

These are my settings:
alias +changec "+attack; cl_crosshaircolor red"
alias -changec "-attack; cl_crosshaircolor 1"
bind "mouse1" "+changec"

If you can't shoot Go ingame and bind the mouse for shoot again!

If im shooting i have my crosshair in red.
And if im not shooting its green.

Thank you for reading my guide.
If you have any problems, feel free to add me :)

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GL HF!