Worked in garter stitch and short rows.
I used
nine shades of green
four shades of yellow
five shades of red

Accidently I found the beginning of a spiral in an old knitting book for knitting machines. Later I learned to build the spiral in garter stitch.

First I worked the green background then the spiral and sewed it on.

This is no pattern but a technique or method. If you want to learn it, go to Heidrun’s Ravelry Swing-Knitting-Site where you can buy tutorials.
The different color fields are knit with a special algorithm. That means: the short rows are shortened in various intervals. Those intervals are combined in a code. The code is developped before starting to knit and will never change until you finish the picture.