php - Make field required only if other field is filled out

I have a form that is a list of products.

ONLY 1 product is required to submit the form.

However, if certain products are selected, a color must be chosen also.

I am having trouble getting the color field required if that specific product is selected.

Here is my HTML code for two of my products.

<input type="checkbox" onClick="return ProductsKeepCount()" name="m13_2" value="BPCK"> B. Polar CoolFit Kit

Must choose color:  
<input type="radio" name="m13_3" value="Khaki"> Khaki 
<input type="radio" name="m13_3" value="Black"> Black

<input type="checkbox" onClick="return ProductsKeepCount()" name="m13_4" value="CSZSV"> C. Steele Zipper Style Vest

Must choose color:  
<input type="radio" name="m13_5" value="Blue"> Blue
<input type="radio" name="m13_5" value="Khaki"> Khaki
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Here is my PHP code that I want to require the color of the product is checked off.

if ($_POST['m13_2']=="BPCK" && && $_POST["m13_3"] == "") 
{
Print("You must choose a color for Product B");
exit();
}
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Any help would be appreciated!