Why A good tin-lead solder joint flows smoothly and solidifies into a bright, shiny, smooth fillet. If the joint is not heated enough, or if it moves while cooling, the solder freezes in a disturbed, grainy state and the surface looks dull, rough, or lumpy. Such a cold joint is mechanically weak and often makes an intermittent or high resistance electrical connection, so it should be reheated and reflowed.
Watch out A bright or shiny surface is exactly what a properly made tin-lead joint looks like, so that choice describes a good joint, not a cold one. Greenish tinge suggests corrosion or flux residue rather than a cold joint.
Shiny and smooth means good; dull, rough and lumpy means cold. Heat it again.