1) Is it allowed for us to use temporary hair color, which is removed with water over time? Its not an obstacle for gusl?
2) If we have kajol in our lower lashes,  makes wazou batil? Is the lashes has to be obligatory wet ?

1) If the hair color has substance to it then it is a barrier for ghusl and wudhu if the hair color is on the scalp. This is because according to Ayatullah Sistani, it is not necessary to wash the hair during ghusl and it is sufficient for the water to reach the scalp. If the scalp only gets the hair color then it is no problem and is no barrier for wudhoo and ghusl. However if the colour substance sits on the scalp, then it would be a barrier for wudhoo and ghusl eg if the hair is dyed using the temporary spray colour then those sit on the scalp and have substance hence being a barrier.

2) If the Kajol sits on the lashes and has substance then yes it would be a barrier. If it's just color then it's not a barrier.