In Texstudio's preamble section where I declare my packages, I have noticed that some of the texts are highlighted as orange.
There are no errors when compiling the document.
Can someone explain this curiosity?
TXS checks all LaTeX commands in the document for validity. This helps you recognizing errors (typos and not loaded packages) already while typing. If a command is marked as unrecognized, TXS did not find a definition for it.
See the above link for more details.