Final answer:
Tia needs to use Form 40N for her Oregon state tax return as a part-year resident, and Colin needs to use Form 40 as a full-year resident. They must file separately.
Step-by-step explanation:
Tia and Colin, who are married and residing in Oregon, filed their federal returns as Married Filing Separately (MFS) and both itemized their deductions. Since Colin is a full-year Oregon (OR) resident and Tia is a part-year OR resident, they need to use different forms for their Oregon state returns.
Tia needs to use Form 40N, which is specifically designed for part-year residents of Oregon. Colin, being a full-year resident, must use Form 40. However, they have to file separately, meaning each of them will file their own individual tax returns.