If you do not want to show any preliminary sketch lines when the painting is completed, find a light gray pastel pencil or light gray charcoal pencil. They blend into the picture and erase easily. Yes, with yellows, they will tarnish the color. Any preliminary sketch will dirty the yellows. For paintings with bright yellows, select the lightest pastel pencil you can see on the canvas.
I don’t recommend the harder pencils that are light, they do not erase as easily (regarding the drafting 4H, 3B, 6B types). If you are doing a specific tonal painting, try finding a neutral colored pastel or one that matches that matches the mass tone of your picture.

I needed to practice some drawing today. It is a boring and tedious process most of the time, but playing around, I discovered a great way to hone in on the skill. Freehand spirographs. Those little pinwheels are hard to do, especially with a technical pen. For those who’ve never played with a spirograph, it looks like this one:
To begin, take a pen, and start at the edge of your imaginary circle. (No fair cheating and draw a circle before hand). Draw a thin oval shape, but before you touch the beginning of the first line, move the pen slightly to produce another oval next to it, meaning //. Do this until you’ve completed your flower. Try to get a good center going.