There is rarely one right answer. Here are several categories of pairing options, each with a reason it may work.
Dark Slate Grey
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Charcoal
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Gunmetal
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Egyptian Blue
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Payne's Grey
Enough lightness contrast to read as two distinct colours without feeling harsh.
Persian Blue
Enough lightness contrast to read as two distinct colours without feeling harsh.
Sapphire
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.
Yellow
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.
Lime
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.
White
Meets WCAG AA contrast (9.0:1) for normal text if used as text/background.
Ivory
Meets WCAG AA contrast (8.9:1) for normal text if used as text/background.
Mint Cream
Meets WCAG AA contrast (8.8:1) for normal text if used as text/background.
Azure
Meets WCAG AA contrast (8.8:1) for normal text if used as text/background.