The most suitable hair color for women around 40 years old depends on various factors such as their skin tone, natural hair color, eye color, personal style, and preferences. However, here are some general guidelines and popular choices that can flatter and enhance the beauty of women in their forties:

1. Enhance your natural color: Embrace your natural hair color or opt for shades that are only a few levels lighter or darker. This can create a sophisticated, age-appropriate look that emphasizes your natural beauty. Add subtle highlights or lowlights to add depth, dimension, and a touch of youthfulness without drastically altering your base color.
2. Consider your skin tone:

• Cool skin tones: Opt for ashy, platinum, or silver blondes, cool browns with taupe or ash undertones, or deep, rich jewel tones like plum or burgundy. These colors can complement your skin's pink, rosy, or bluish undertones.
• Warm skin tones: Choose golden, honey, or caramel blondes, warm browns with reddish or golden undertones, or rich auburn shades. These hues will harmonize well with your skin's yellow, golden, or olive undertones.

1. Soft, dimensional highlights: Soft, face-framing highlights or balayage can brighten your overall appearance and create a youthful, radiant glow. Opt for shades that are 1-2 levels lighter than your natural color or complementary tones that blend seamlessly with your base.
2. Subtle ombré or sombré: Graduated color techniques like ombré or sombré, where the hair gradually transitions from a darker root to a lighter end, can add dimension and interest without looking overly dramatic or high-maintenance. Choose colors that complement your natural hair color and skin tone.
3. Gray blending or embracing: If you have started to develop gray hair, consider incorporating it into your overall look. You can opt for gray-blending techniques that seamlessly mix your natural grays with your chosen hair color, or embrace your grays entirely by transitioning to a stunning, all-over silver or salt-and-pepper style.
4. Update your hair color regularly: As you age, your skin tone and natural hair color may change. Regularly reassess your hair color and update it as needed to ensure it continues to complement your evolving features.

Remember, the most suitable hair color for a woman in her forties is one that makes her feel confident, beautiful, and true to herself. Consult with a professional hairstylist who can help you choose the perfect shade based on your unique features, lifestyle, and personal preferences.