Step 1: Recall the basis of Crystal Field Theory.
Crystal Field Theory treats the bond between a central metal ion and its ligands as a purely electrostatic attraction, with ligands acting as point charges or point dipoles. We test each listed item against what this theory can explain.
Step 2: Test statement II, formation and structures of complexes.
CFT explains how the $d$ orbitals split in octahedral and tetrahedral fields and so accounts for the geometries and relative stabilities of complexes. So II is explained.
Step 3: Test statement III, colour.
The splitting of $d$ orbitals creates an energy gap; electrons jumping across this gap absorb visible light, which gives the complex its colour. So III is explained.
Step 4: Test statement IV, magnetic properties.
Depending on the size of the splitting, complexes are high spin or low spin, which fixes the number of unpaired electrons and the magnetic behaviour. So IV is explained.
Step 5: Test statements I and V.
Treating ligands as point charges (I) is an assumption of the model, not a property it explains, and covalent character of metal-ligand bonding (V) is exactly what CFT ignores since it is purely electrostatic. So I and V are not successes of CFT.
Step 6: Conclude.
The properties CFT successfully explains are II, III and IV. The answer is
\[ \boxed{II,\; III,\; IV\ \text{only}} \]