If you're using standard in-game customization options, then it will happen by default. Your soldiers keep the colors and patterns you've chosen but the armor itself changes. If you are using modded cosmetics and have something like Unrestricted Customization it locks the look to whatever you have set regardless of armor tier. Another option is to use Appearance Manager, there you can individually customize each armor tier for each soldier, or create uniforms to apply with just a couple clicks.