Painting folds in fabric and cloth?
For instance as the fold moves in and out of the light, the colours change from warm to cool and often light to darker. Use of complementary colours (ie red/green, blue/orange, purple/yellow) will help you describe these undulations in a realistic and three dimensional way. Take a look at the attached image and you will see how I have used variations of orange/blue and purple/yellow to establish the folds. See how they glow with colour? Even though this little dress 'reads' cream - in fact there are so many jewel-like colours within this painting of fabric folds - they all describe the colour cream but there is very little that is just a simple cream hue.
So you see, painting folds - as with everything - is really about observation and even enhancing what you see if it emphasizes the atmosphere you wish to create.
In this instance I wanted to give the impression of late afternoon sunlight filtering through this fabric as the little girl danced amongst the daffodils picking her favourite ones for her mother.