Thanks for reaching out to us in order to find out about the reason your beloved Samsung Galaxy S9+ has yellowish screen and as per your observation may have something to do with a weak battery and we are sorry to hear that.
I feel and understand why you are worried for a device that you've take care so much. If my device is facing similar experience I will be bothered too.
Before you try out the below recommendations as to your solution, be sure to check if your device's software are updated with the latest version. Please check how to update your mobile device's software in the following order.
Step 1. Go to Settings > Software update.
Step 2. Tap on Download and install.
Step 3. Follow the on-screen instructions.
Use Diagnostics in the Samsung Members app on your Galaxy phone
You can perform tests to make sure your phone is working properly with Diagnostics in the Samsung Members app. Follow the steps below to run Diagnostics via Samsung Members.
Step 1. Launch the Samsung Members app, and then tap on the Plus (+) icon for more options. Alternatively, tap Get help.
Step 2.SelectDiagnostics.
Step 3. If you would like to run through all Diagnostic checks please tap on Start.
Step 4.Your Galaxy phone will then run through the checks. Then follow the on-screen instructions to complete the check.
Step 5.If you prefer to just check one specific setting, simply tap on the setting to begin. You may check and tap both Battery and Display.
If the result show there is something problem with the battery, then I highly recommend bringing your device to the nearest Samsung Authorized Service Center in your area.
Rest assured that your device will undergo an assessment to pinpoint the root cause of the issue. They may guide you and offer solutions to restore it.
