I'm trying to be an expert for choosing colors and combining colors. So I found a tutorial making schools sets icons and for a start, I am making a pencil but I'm not going to follow the same exact color on the tutorial. I'll do it my own with a touch of red with the flat pencil vector as an exercise for me, I am choosing the monochromatic method of color theory but how do you know exactly if your lighting or shadowing of red on the pencil is correct or industry standards? I am having the hard time understanding people I encountered in youtube on how they choose a light or shadow color of the main color. If there are any question please free to ask. Thank very much.
Which color is best to use is a matter of opinion although there are many resources online to help you find color pallets and color schemes for certain purposes. I can help you find the shading for a color though. To make a color look naturally blended to light and dark you should find the tint and shade of that color and use them as the highlight and the shadow. A tint is a color with white. A shade is a color with black. Open your color pallet with the main color selected (Note the hex value of your main color). Go straight up from the main color and click to find your tint. Make note of the hex value. Go straight down from your main color and select the shade. Note the hex value. You now have three colors, your main one, a highlight and a shadow. Apply them to the three sides of your pencil for the desired effect, or put them in a gradient for a smooth round colors surface. You must decide how far to go up and down for your colors.
Smashing magazine have sevral good articles about the "Theory of Color" I recommend reading this 3 parts series from the beginning, but to answer your question you can jump to part 3: https://www.smashingmagazine.com/2010/02/color-theory-for-designer-part-3-creating-your-own-color-palettes/