Can I be sued for an incomplete job if the client did not like the color of the stained concrete they chose and will not let me finish?

She could potentially sue you for a refund.  However, you will have several viable defenses.  First, you didn't have a written contract that set out a refund condition for dissatisfaction.  Second, you did perform as instructed by a person with authority, namely the assistant who approved the color.  You're not liable for a miscommunication between her and her assistant.  Third, even though you don't have a contract, you do have some evidence as to what the agreement was--namely the estimate form.  Fourth, you did attempt to complete the project-- so she can't fault you for non-performance when she intentionally interfered with your performance.  If possible, you may want to refund any unused and unearned portions (if any) of the deposit that she gave you.  For example, if you can return the sealer for full price, then do so and give her that portion back.   You shouldn't have to return any earned portions or fronted expenses that you cannot recoup.  If you're going to continue to do jobs on the side, you may want to visit with an attorney that has experience handling contracts and have them draft you a standardized form contract for your business that gives you more concrete protection to avoid a repeat problem in the future.