Question. is Water Yin Or Yang.?
Answer.    It can be either, it depends how and what you reference it to   :-

e.g. consider the process of
[1] Ice melting to Water and then .......
[2]Water turning to steam.

In [1] Ice (cold, solid) is more Yin compared with Water (warmer, fluid) which is more Yang
In [2]Water (fluid) is more Yin  compared with Steam (light, vapour) which is more Yang

There are no limits to the scale Of Yin & Yang
i.e. In Gross terms
Infinity - universe( more Yang  ) to almost zero -one solitary electron (more  Yin )
Or in more minimal terms :-
a rise in temperature from 20 Degrees (more  Yin ) to 20.00000001 Degrees (more Yang )

In general usable terms:-

Yin relates more to Coolness (  the most Yin  state of water is ice), ovum, earth, rest, darkness (Black), inward, concentrated, Acidic.
Yin meridians are on the front of the body & inner surfaces of the limbs.
They are relatively solid organs (kidneys, liver, heart, heart, heart governor, spleen, lungs).
Yin organs are deeper, internal and deal with the transformation, circulation, and storage of Qi and blood.
Yin emotional trends are spiritual, open, peaceful, joyful, creative, colourful, tolerant, non-conformist (hippy like).
Yin arrogance - the feeling that no one can help them.

Yang relates more to Warmth ( most Yang  state of water is steam), sperm, heaven, activity, brightness (white), outward ,expanding, Alkaline.
Yang meridians are on the Back of the body & outer surfaces of the limbs. They are relatively hollow organs (bladder, gall bladder, small intestines, triple heater, stomach, large intestines).
Yang organs are more surface including the digestive tract that deals with receiving / processing food and eliminating waste.
Yang human tendencies are decisive, purposeful, concentrated, clean, orderly, polite, (yuppy like).
Yang arrogance - if you try to give them help they know better.
 

Applying Yin & Yang theory to diet means choosing a balanced diet of both Yin & Yang  foods which compliment each other.

Yang  Foods like Beans & Cereals should be balanced with more-
Yin    Foods like Sea & Land vegetables.

At the extremes:-
Very  Yang foods include Red Meat, eggs, salt, cheese.
Very  Yin foods include Refined foods (sugar), alcohol, ice cream, fruit.

Headaches also arise from the liver Yang creating excess Heat also impairing the ability of the Liver to clean the blood efficiently, leaving toxins to accumulate in the Blood.

Headaches also stem from misalignment of vertebrae which stress's the nerves, also bad posture, sustained noise, toxic smells & overworked eyes.
 

Types of head-aches	Sharp Headaches are Yang in nature Dull Headaches are Yin  in nature Pounding/ Throbbing Headaches are an indicating of Liver Yang rising.
Anterior (Frontal) (Yin)	This affects the area around the forehead, eyes and sinus's also  including the third eye. Eating excess Yin food like sugar, ice cream, alcohol contribute to the intensity of the headache, often accompanied by constipation. The associated meridian organs are Stomach and Bladder. For Local relief use B2,GB14 Distal relief use LI4 General St36 and Lv3
Posterior (Back) of head (Yang)	This affects the area of the back of the neck and head. Eating to much Yang food like meat cheese, eggs, salt, contribute to the intensity of the headache. The associated meridian organs are Bladder - Kidney and Spleen For Local relief use GB20 For Distal relief use LI4&BL60 General relief ST36
Lateral (Side) Headaches	Affecting the side of the head and around ears. The associated meridian organs are Liver & Gallbladder and Triple heater. For local relief GB20, TH5 For Distal relief TH4, GB34, Bl60 General ST36 & Lv3
Migraine	Affecting more inside the head, could be just left side only (bad lymph) or right side only (too much gall), associated meridian organs liver and gallbladder, early signs are Dark alkaline urine (which indicates rising heat), pain in the eyes and yawning. Yin       foods like Honey, fruit (sugar), and inhalation of vinegar (acid) fumes help. Yang  foods like meat and dairy produce should be avoided. Local Relief B10.

From the viewpoint of TCM the Livers function of storing blood pays an importantly role in the well being of the female reproductive system, also exposure to cold (Yin ), excess cold food, weakness of Qi and blood are the major causes of menstrual disharmonies.

The reproductive system is regarded as being Yang  in nature, so excess Yang influences is damaging to the system, i.e. Eating excess Yang  food (meat, eggs, salt), taking too hot baths, (a daikon bath is good for strengthening the reproductive system).
 

Pain before period	Feelings of irritability, depression, frustration accompanied with pain, Traditional Chinese Medicine (TCM) diagnoses this condition as stagnation of Liver Qi, the liver is Yin in nature however it regulates the Yang  Blood flow in the body process's, deficiency of liver blood may lead to lack of periods.
Pain During Period	Often severe pain in lower abdomen, heavy blood flow and clots, in TCM terms this indicates a Liver-Spleen disharmony, the liver can also transmit too much heat into the blood causing excessive menstrual bleeding and bleeding between periods.
Pain After Period	Pain is better with application of warmth and pressure. Scanty blood flow, palpitations often accompany the pain, in TCM terms this indicates a Qi and blood weakness.
Heavy Menstruation	Feelings of restlessness and with deep red Menstrual Blood, in TCM terms it is a liver Qi stagnation which creates heat and excess flow of blood, often caused by emotional stress. If the blood is light-coloured then it is a indication that the spleen is not controlling the blood in the vessels caused by worry or poor diet.
Pre Menstrual Syndrome [PMS]	Symptoms of Irritability, depression, nausea & Lower abdominal pain, in TCM terms this indicates  Liver Qi Stagnation affecting Spleen & Stomach causing Qi to rise in the Stomach Channel.
Menopause	Symptoms of Tiredness, Hot Flushes, irritability, insomnia & palpitations, in TCM this is imbalance of Kidney Qi, creating a  Yin / Yang  imbalance in the Blood & Qi which affects the Heart, spleen & liver, the Kidney  Yin should be nourished.