Painting folds with acrylics is easiest if you do some preparatory work on your palette first.
Take a good look at the folds and premix some main colors/tonal values.
In this example I’ve premixed three tonal values of red: one dark value, one midtone and one light value.
Now you can start painting! Just block in the dark values first. Even though you may see some nuances in the dark values, just block in the one color that you premixed. If necessary, you can make adjustments later on.
Then block in the midtones. Don’t mind blending the values at first. First put the midtones at the right spots.
After that, you can mix a bit of the midtone and the darker value on your palette. This is a transitional value that you can put on the boundary between the two tonal values and now you can easily mix them.
Now repeat the process for the lightest value.
Maybe your painting looks ugly at this stage, don’t worry about that. In this case for instance, the painting needs a second layer, because of the transparancy of the pigments.
After repeating the previous steps for all values, the painting looks more logical: