Change sub-menu css class

if you don’t want to use a custom menu walker, you can use the classes wp already puts out.

If you add a class to the menu item you know will have a extra ul, you’ll have something like this

<ul>
    <li class="menu-item menu-item-1">Item 1</li>
    <li class="menu-item menu-item-2">Item 2</li>
    <li class="sub-1 menu-item menu-item-3">Item 3
      <ul class="sub-menu">
          <li>Sub menu item 1</li>
          <li>Sub menu item 1</li>
      </ul>
    </li>
    <li class="sub-2 menu-item menu-item-3">Item 3
      <ul class="sub-menu">
          <li>Sub menu item 1</li>
          <li>Sub menu item 1</li>
      </ul>
    </li>
</ul>

you can then select in your css

.sub-1 ul { color: blue; }
.sub-2 ul { color: red; }

to add a css class to the menu item, make sure you have “css classes” ticked, in the “screen options” in the menu page.