The selector changes to blue, but the product image remains red. First check whether the specific variation has its own image and exists as an actual combination. Adding an attribute value does not automatically create every possible variation.
For a fictional mug, record colour, size and the expected image file. An internal variation ID alone is not a useful description of the intended visible selection.
Check data before changing the theme
Open the variation in the product editor. Compare its image, price and availability with the parent product. A variation without its own image presents a different issue from a missing or unavailable combination.
Open the public product page in a fresh session. Select every required attribute, then change only the colour while observing the image and availability message. Return to the original choice. If caching is involved, isolate it deliberately on a test environment rather than clearing the entire shop state without a diagnosis.
Add the available test variation to the basket without completing an order. Its name, properties and image should convey the actual choice. After a theme change, repeat the task with another product whose variation data is known to be correct.
Record data and presentation issues separately. Add a missing image mapping in the product editor. If correct variation data still does not produce an image change, inspect the selection handler in the theme.