There is rarely one right answer. Here are several categories of pairing options, each with a reason it may work.
Chambray
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Steel Blue
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Slate Grey
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Royal Blue
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Cerulean
Enough lightness contrast to read as two distinct colours without feeling harsh.
Payne's Grey
Enough lightness contrast to read as two distinct colours without feeling harsh.
Denim Blue
Enough lightness contrast to read as two distinct colours without feeling harsh.
Periwinkle
Enough lightness contrast to read as two distinct colours without feeling harsh.
Grey
A low-chroma neutral lets the original colour stay the visual focus.
Taupe Grey
A low-chroma neutral lets the original colour stay the visual focus.
Silver
A low-chroma neutral lets the original colour stay the visual focus.
Ash Grey
A low-chroma neutral lets the original colour stay the visual focus.
Yellow
A saturated accent from a different hue family for a deliberate pop of contrast.
Gold
A saturated accent from a different hue family for a deliberate pop of contrast.
Chartreuse
A saturated accent from a different hue family for a deliberate pop of contrast.
Citrine
A saturated accent from a different hue family for a deliberate pop of contrast.