Firstly, sapphire and ruby both belong to the the family of corundum, which comes in all colours. All colours except red are referred to as sapphire.
Corundum in its purest state is aluminium oxide (Al2O3), which is a colourless sapphire. Additional elements or impurities combined with these base elements gives the breathtaking sapphire colors.
Combination of titanium and iron makes a blue sapphire. Chromium impurities make a ruby red - various shades of pink to red are based on the amount of chromium.
Existence of chromium with other elements of a blue sapphire makes a purple sapphire. Yellow sapphires are a bit more complicated. They have a few different ways of getting their color. Majority are colored by the existence of iron (I won’t discuss other methods here as it involves a lot more chemistry). Iron is also responsible for green sapphires; due to rarity green sapphires are virtually unheard of.As you can see iron is responsible for blue, yellow and green colors. Majority of Australian blue sapphires tend to have more of a greenish secondary hue mainly due to higher quantities of iron.
Other than the color, all other properties such as hardness and toughness remains the same across all sapphire colors.
We are also working on updating the website with a more descriptive content around sapphire colors. Until then hope you enjoy reading.