Why should you never use a pen, but rather a pencil, to mark your starting line on a tlc plate?

You can't use a pen because the ink will travel up the TLC plate with the TLC solvent, just like your chemical samples do.
You can only spot/label/mark TLC plates using a pencil. The graphite in a pencil will not run up the plate!

He could go with d. temperatempera coloring is very precise and can easily be used if you have the skills necessary to paint hair. this was commonly seen in religious paintings made with tempera techniques. the first tree are not that much about fine details and photorealism and are more about vivid colors and expressiveness.