You will pay income tax to the state of California. Washington state does not have an income tax so you do not need to file a return with them. Since you were living in California while you were performing the work, your income is considered to be sourced from there and is taxable by California.
If California is not where you are permanently residing and you have an established residence in another state, you may have to file an income tax return for that state as well. If you do, you should receive a credit for the taxes paid to California on this income.
