It is quite rare for the iris - the coloured part of the eye - to change colour. Sometimes, your eyes may seem to have changed colour due to different lighting conditions making them appear lighter or darker, but this is usually not the case. Eye colour is determined by the distribution and concentration of the pigment melanin.

For example, babies are typically born with blue eyes, but they darken in the first three months as more melanin develops. Changing hormone levels, such as during puberty or pregnancy, may be linked to slight variations in eye colour, although this will be limited and non-permanent. Gradual changes in the colour of the eye may also be due to the natural ageing process and are usually nothing to worry about.

There is a condition called heterochromia, which is where your eyes have different colours. This may mean two different coloured eyes - such as one brown and one blue - or having two or more colours within the same eye. Many celebrities have heterochromia, such as Mila Kunis, Kate Bosworth and Christopher Walken, and it can be genetic or result from an infection or trauma. If heterochromia appears suddenly and is not inherited, you should book an appointment with an optometrist, as there are some rare conditions and diseases that can cause this to happen. These include:

• Fuchs heterochromic iridocyclitis
• Horner's Syndrome
• Pigmentary glaucoma
• Tumours
• Duane Syndrome

These conditions, while uncommon, could cause long-term damage to your eyes, vision and overall health. It is therefore important to seek medical attention as soon as possible.

Can I change the colour of my eyes?

There are a number of commonly cited old wives' tales regarding changing your eye colour, many of which are not true. For example, some people claim that a mixture of honey and water can lighten your iris when applied to the eye. Not only is this not true, you could risk an eye infection, so it is definitely not recommended. Others have suggested that blowing your nose hard enough has resulted in their eye colour changing. Again, this is not true. Currently, there are no known treatments for changing your eye colour aside from wearing coloured contact lenses. It is important to get these fitted by an optometrist to ensure you purchase the right lenses for your eyes.