BLUE;is a recessive colour and one of the primary colours and it gives the impression that it is receding optically. Useful for backgrounds in landscape paintings.Blue and red makes purple.

There are at least seven kinds of blue

(1) Azurite blue also known as Ceruleon Blue;

(2) Cerulean  Blue also known as Azurite Blue;

(3) Cobalt Blue;Useful for creating the feeling of recession in landscapes(It is similar to Sian Blue)

 (4)French Utramarine Blue; This is a rich dark blue and a good mixer. If you mix it with burnt umber it forms a rich black. It is also good for recession impressions in landscape paintings..  

(5) Phthalo Blue;

(6) Sian Blue; is not available in oil paint.(it is similar to Cobalt Blue)

(7) Ultramarine Blue; Mix it with sap green and you get a misty grey green. this can be used for springtime foliage.

Warning tip; Do not use linseed oil with whites or blues as it has  a tendency to yellowYou can use poppy oil but it is a slow dryer