Thrust vector control on the Titan solid rocket motors was accomplished by fluid injection rather than gimbaling the nozzles. Nitrogen tetroxide oxidizer was injected into ports around the circumference of the nozzle to alter the direction of the exhaust and control the vehicle.
The red tank contained the nitrogen tetroxide propellant used in this thrust vector control method.
THRUST VECTOR CONTROL SYSTEM: The TVC is used to maintain a proper vehicle flight path and orientation during the Stage 0 portion of the mission trajectory via signals from the Core Vehicle guidance computer. The desired flight profile is achieved by exerting side forces on each SRM by injecting a flow of gas, at a specified angle, at a defined velocity and for a calculated time into the SRM gas flow stream through the nozzle. The SRM nozzles are at a fixed angle and do not gimbal.
is provided by a Thrust VectorControl
from the Titan
control computer. Nitrogen tetroxide injected
the SRM nozzle through TVC
Pressurized tanks attached
thrust vector control
for the TVC system.
Reference: NASA TM X-68249
Drawings from TITAN
In regards to your comment about asymmetry of the booster, each SRM had a tank, mounted on opposite sides of the vehicle, so it's not as bad as it looks from that photo.
This thrust vector control method has been used by at least one other launcher, the Indian Space Research Organization's Polar Satellite Launch Vehicle. See Ohsin's answer at this question How does a single SRB control attitude? for more information.