Have you tried the Good Lock module, which is made by Samsung and can be used to customise many parts of your phones UI including the volume sliders.
For the volume sliders download Good Lock from the Galaxy Store. Once downloaded open the app and select the Family tab, from this list there will be an item called Theme Park... this is the one you need to customize the UI colours.
Good Lock Link: https://apps.samsung.com/appquery/appDetail.as?appId=com.samsung.android.goodlock
On my S10+ powered by Android 10 improved by One UI 2.0,