Read the FTB publication and see if you will be considered a California resident. If you are a nonresident of California, you still have to pay California income tax on the money that you earn for the time that you work in California. You will have to file a California nonresident tax return. Your employer is supposed to keep track of the amount of time that you work in each state, and report your California income separately in the state section of your W-2. They wouldn't normally show your Washington income on the W-2, since there is no Washington income tax. So California doesn't determine how many days you worked in California. Your employer tells them how much California income you had.
If your employer does not have any other employees in California, they might not want to report your California income because they would have to register as a California employer, withhold California taxes from your pay, pay California payroll taxes, and probably pay other California business taxes. But legally they are required to report how much they pay you for working in California.
Investment income, such as dividends and interest, is treated as being from the state that you live in when you receive the income. So if you are not a California resident, you would not have to pay California tax on your dividends and interest.
