Scandium is not considered a transition metal because its most stable ion has no d-electrons. A transition metal is an element that forms an ion with an incomplete d subshell. Scandium's most stable ion is Sc3+, which has no electrons in its d orbitals.
Explanation
Scandium's electronic configuration is
Ar 3d1 4s2
To form the
Sc3+ ion, scandium loses the two electrons in its 4s shell and one electron in its 3d shell.
The
Sc3+ ion is colourless because it has an empty 3d sub-shell, which prevents electronic transitions that produce colour.