Every time you wish to put more thought into adding something to your phone you run a risk of affecting your device’s performance. You sometimes sway away from incorporating any app or any setting because you want your device to run without any issue. You are actually not affecting your device’s performance by customizing your app icons to grayscale or inverted colors because they are native to iOS’s Accessibility settings. Also, when changing the app icons through Shortcuts, you aren’t affecting the performance because it just provides an additional shortcut to the preferred application and doesn’t affect the original app tile. However, there’s no practical data on the third-party applications Color Widget and Widgetsmith so you can’t be sure if they affect the performance of your device.