There is rarely one right answer. Here are several categories of pairing options, each with a reason it may work.
Amethyst
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Slate Blue
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Fuchsia
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Chambray
Similar lightness for a subtle, tone-on-tone pairing with minimal contrast.
Slate Grey
Enough lightness contrast to read as two distinct colours without feeling harsh.
Dark Orchid
Enough lightness contrast to read as two distinct colours without feeling harsh.
Storm Grey
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.
Prussian Blue
Strong lightness contrast and saturation for a bold, graphic pairing.
Tyrian Purple
Strong lightness contrast and saturation for a bold, graphic pairing.
Cinnamon
Strong lightness contrast and saturation for a bold, graphic pairing.
Burgundy
Strong lightness contrast and saturation for a bold, graphic pairing.
Taupe Grey
A low-chroma neutral lets the original colour stay the visual focus.
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.
Lavender
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.
Yellow-Green
A saturated accent from a different hue family for a deliberate pop of contrast.
Peridot
A saturated accent from a different hue family for a deliberate pop of contrast.
Avocado
A saturated accent from a different hue family for a deliberate pop of contrast.