Occasionally, I do not tell my husband the truth to avoid his anger. Is this considered grave matter?
It would depend on the severity of the issue at hand. For example, denying having bought a new lipstick would be much less grave than denying having bought a new car. If you have trouble discerning the gravity of the action, ask your confessor or spiritual director for assistance.
While it is true that spouses sometimes employ mental reservation or avoid discussing certain issues in order to keep family peace, I cannot recommend deliberately lying to your husband. The temporary peace you buy with a lie will be far outweighed by the damage to your marriage if he finds out that he cannot trust you to be honest with him.
If marital honesty and fear of anger continue to be problem areas in your marriage, you may wish to suggest to your husband that you seek assistance from a Catholic-friendly marital counselor. For referrals to a practitioner in your area, I recommend the Pastoral Solutions Institute and CatholicTherapists.com.