The concentration of Aluminium Sulphate in the bath will slowly build up. Replace the acid solution from time to time. A damaged anodised finish can be removed by VERY brief immersion in a strong caustic soda solution. Once having sealed up the oxide film it is not possible to increase the film thickness by reimmersing it in the acid bath because of the high electrical resistance of the oxide layer. Dyes for cotton fabric are satisfactory but don't add mordants or anything to the dye, e.g. salt, vinegar etc.
Vegetable dyes are reputedly better than chemical ones.
Most colours can be used at 20gms of dye per 600ml of tap water. Black needs to be much stronger at 60gms per 600ml or thereabouts. Place into the cold dye bath without allowing it to dry out. Raise temperature of dye bath to boiling point and allow it to boil for ten minutes. Remove workpiece and hang it in clean boiling water for 15 minutes.