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 Sc\(^{3+}\), which has no electrons in its d orbitals. 

Explanation

• Scandium's electronic configuration is Ar 3d\(^1\) 4s\(^2\)
• To form the Sc\(^{3+}\) ion, scandium loses the two electrons in its 4s shell and one electron in its 3d shell. 
• The Sc\(^{3+}\) ion is colourless because it has an empty 3d sub-shell, which prevents electronic transitions that produce colour.