Their colour is reliant on soil pH. More specifically, an enzyme in the plant called the delphinidin-3-glucoside-aluminum-3-caffeoylquinic acid complex. 

Aluminum makes the flowers of Hydrangeas turn blue. But, Hydrangea plants can't absorb Aluminum through their roots at a pH above 6. It is bound too tightly to the soil particles.

 

When we lower the pH and make the soil more acidic, the bonds between the soil and Aluminum weaken, making it available to Hydrangeas. They absorb the aluminum which activates the enzyme we talked about above, and the flowers turn blue. When the Aluminum isn't present in the plant though, the enzyme isn't activated and the flowers remain pink.

Vitamin D promotes the absorption of calcium and phosphorus from the gastrointestinal tract. A deficiency of vitamin D makes it difficult to maintain proper calcium and phosphorus levels in bones, which can cause rickets.

Adding vitamin D or calcium to the diet generally corrects the bone problems associated with rickets. When rickets are due to another underlying medical problem, your child may need additional medications or other treatment. Some skeletal deformities caused by rickets may require corrective surgery.

 The albino mutation in Siamese cats results in a defective form of tyrosinase which does not function at normal body temperature. Therefore, dark coloration can only appear in parts of the body that are cooler than the core body temperature. The extremities are always the coolest parts of the body. The face is also cooler because of air passing through the sinuses. The back is warmer than the extremities, being closer to the body core, but it is also exposed. The result is a medium degree of tyrosinase function, resulting in a medium degree of shading

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