Most anatomical illustrations show a simple use of colours as follows:

• Red for the heart and blood system.
• Blue for the veins and lungs.
• Buff for skin and eponurosis, cartilages and bones.
• Grey and brown for the bladder and other organs.
• Yellow for nerves.
• Dark brown for the spleen.
• Pink for the stomach.

An excellent interactive example can be seen at Merck Source™. Simply click the following link and choose "Interactive Body Guide" from the index shown. Launch Website.

Source:
FURNEAUX, W.S. (c.1900) Philips' Popular Mannikin. London: George Philip & Son.