Soul
Pikmin Spirits will only appear if a Pikmin was killed by ahazard or was eaten by an enemy. They roughly resemble the contours of Pikmin; however, they lack feet and leaf. They are also slightly transparent.
In Pikmin 1, every Pikmin will have a same coloured blue spirit, irrelevant of the colour of the Pikmin itself.
Spirits in Pikmin 2 show the colour of the Pikmin skin. They also differ in sizes; this can be seen especially well with Purple and White Pikmin.
Enemies
Enemies spirits have a pink-purple colour and look like a bubble with a more transparent "tail" trailing behind it. They show two holes which resemble the eyes. Bulbmin have a smaller version of this type of spirit.
Also, there is a minor glitch involved with enemy spirits. If you kill an enemy and let the corpse off screen, the enemy spirit will not show until it is put back on screen.