Oddly enough, we have another 4K TV with built in Roku, the colors are fine on that TV using Spectrum app on Roku.
Your comment about the second TV suggests that the service and app are working correctly.
This points to a possible configuration or set up issue with the Apple TV. On our test equipment, we have found that the picture is optimized when we select "Match Content" as follows (from Apple TV Support):
Go to Settings > Video and Audio > Match Content, then choose from the following:
If you turn on either of these settings, Apple TV will attempt to automatically determine which dynamic ranges or frame rates your television supports.
If Apple TV can't determine which display formats are available, you need to verify which formats your television can display. To verify a format, go to Settings > Video and Audio > Match Content. Then select a format under Unverified Formats to run a short display test and verify that it displays correctly on your television.