How to read the band →
Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Colby
Colby is now well below its former popularity. All-time it's been given to 75,255 U.S. babies, the 675th most common name by sound today, peaking at 4,790 in 2001, with 1,032 in 2025.
By sound, Colby also includes the spellings Kolby, Colbie.
Rising
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).