What is the main difference between a true solution and a colloidal solution in terms of stability and appearance?

A true solution is transparent and stable; its particles do not settle down even on prolonged standing. The solute particles are extremely small (less than 1 nm) and invisible. For example, sugar solution. 

A colloidal solution (or colloid) is generally translucent and appears homogeneous but is actually heterogeneous. Its particles are larger than those in a true solution (1-100 nm) but too small to be seen with the naked eye and do not settle down. Colloids are also stable, meaning their particles do not settle. 

However, they scatter light (Tyndall effect), unlike true solutions.