Is there any way to change my Contacts and Google Phone theme back to the original darker colors? They changed a few days ago to a washed out selection of light pastels.
Welcome to Android Central! Which phone do you have? When Google modifies the design language, you're stuck with how it looks -- there's generally no way for the user to adjust it (apart from the eternally promised Dark Mode). You'd have to consider a 3rd party contacts or dialer app that has a theming option.
There's no easy switch you can flip to change to the old look or to a darker theme, though the Phone app supposedly has a dark mode that is "coming soon". In Contacts you do have the option to edit each contact's avatar/icon so it's something other than just a pastel circle.
The other option would be to download and install an older apk from www.apkmirror.com.