"Experience" is more than just "interface". E.g. which actions are lightning-fast, and which are painfully slow is an important part of user experience, even if the UI is exactly the same. Performance and space limitations, things like limited / unlimited undo, interoperability with other software / supported data formats, etc are all important parts of UX that are not UI.
UI, where I stands for "interface" just like in HCI, used to mean all those things.
But in the industry the focus turned to aesthetics, so a new term was invented to differentiate between focusing on the entire interface ("experience") vs just the look.
Just like "design" encompasses all of it, but we add qualifiers to ensure it's not misunderstood for "pretty".
Thing is: changing the colours _could_ be improving the UX.
Eg I'm colourblind, and a careful revision of a colourscheme can make my life easier. (Though I would suggest also using other attributes to differentiate like size, placement, texture, saturation, brightness etc.)
