I have an LLC registered in CO, I would like to register a DBA in CA. How is this legally done?

Most municipalities will allow for this. Call you local city offices or county offices for direction, IF you cannot find this info online.

If the LLC wishes to do business in CA you must register the LLC with the CA Secretary of State. DBAs are typically registered in the county where the LLC's physical presence in CA is located.

If your Colorado LLC intends to have a physical presence in California or do business in California it needs to register as a "foreign" LLC doing busines here. That California LLC once registered files a dba just like any "domestic" (California formed) LLC>

Gennerally DBAs are filed so that your business can use a trade name that is different from the LLCs legal name (designated in the Certificate of Formation for LLCs). If your LLC will be using its actual name, there is no need to file a DBA. As other counsel have suggested, you should determine if your LLC will be required to register and qualify as a foreign entity engaging in business in California. This determination depends on the number of transactions and type of business your Colorado LLC will engage in herein California. For example, if your CO LLC will simply be holding title to real estate or another asset in California, it probably will not be required to register. But if your LLC will have a business location or California based employees for an ongoing venture, then you will need to register with the Secretary of State as a Foreign LLC and pay the annual fees and taxes to maintain authorized status.