Blue, cobalt, ultramarine, pthalo blue, azure, cerulean, indigo, so many names for blue. When you choose colours to paint with you have to select the right one. When you choose you may want a transparent blue, or something stronger with a real depth of colour to it. If you want a colour to paint the sky you need to match what you see to what you can mix on the canvas. Sometimes you just can’t get the right combination of colours. Some blues have hints of red, or purple, or green or even yellow, it’s amazing how many variations of colours the eye can see. Sea, world, ocean, sky, so many blues.