function hideStock(){
var selected = document.getElementById("stk");
var hidden = document.getElementById("pa_color");
if (selected.onchange=="stock") {
hidden.style.display = "none";
}
}<table class="variations" cellspacing="0">
<tbody>
<tr>
<td class="label"><label for="pa_pricing">Pricing</label></td>
<td class="value">
<select id="pa_pricing" class="" name="attribute_pa_pricing" data-attribute_name="attribute_pa_pricing" data-show_option_none="yes" onchange= "getSelectValue(this.value);">
<option value="">Choose an option</option>
<option value="retail" >Retail</option>
<option value="stock" id = "stk" >Stock</option>
</select>
</td>
</tr>
<tr>
<td class="label"><label for="pa_color">Color</label></td>
<td class="value">
<select id="pa_color" class=" " name="attribute_pa_color" data-attribute_name="attribute_pa_color" data-show_option_none="yes">
<option value="" >Choose an option</option>
<option value="aqua" >Aqua</option>
<option value="black" >Black</option>
<option value="mixed" >Mixed</option>
</select>
<a class="reset_variations" href="#">Clear</a>
</td>
</tr>
</tbody>
</table>try it:
var selected = document.getElementById("stk");
var hidden = document.getElementById("pa_color");
selected.addEventListener('change', _ => {
if (selected.value == "stock") {
hidden.style.display = "none";
}
}