This popular vote for United States Senator was technically only an advisory vote as prior to the adoption of the 17th Amendment in 1913, state legislatures officially elected United States Senators. By the time of the election in November 1912, the amendment had already been passed by Congress, but not yet ratified by the states. The Colorado General Assembly ratified Shafroth's election on January 14, 1913.
Source for Popular Vote data: Colorado Secretary of State. Abstract of Votes Cast at a General Election Held on Tuesday, November 5th, A.D. 1912, for United States Senator, Colorado Secretary of State (https://www.sos.state.co.us/pubs/elections/Results/Abstract/pdf/1900-1999/1912AbstractBook.pdf) (accessed 21 Sep 2023)