Show unchecked checkbox(s) with a different color in mailto

I'm doing a form that is supposed to send its answers to an email. It does send the checked answers, but i would like to have the unchecked answers in red for example in order to show what's checked and what's not in the mail.

<form action="mailto:myadress@gmail.com?subject=New mail" name="email" method=POST enctype="text/plain">
<input type="checkbox" id="check1" name="check1" value="Check_1">
<label for="coding">Check 1</label>
<br/>
<input type="checkbox" id="check2" name="check2" value="Check_2">
<label for="coding">Check 2</label>
<br/>
<input type="checkbox" id="check3" name="check3" value="Check_3">
<label for="coding">Check 3</label>
<br />
<input type="submit" value="Send">
</form>

If someones could help me with this problem,

Thank you

For example, the mail would be

check1=Check_1

check2=Check_2

check3=Check_3

with red font instead of italic