My cleric's max weight is 90 (her str is 12) but when she's carrying about 70 pounds the numbers get red color. below 70 it's yellow. when it's read will she have any penalty?

I don't think so, it's just to warn you that you are getting closer to the max.

Weight will make the character will move slower after a point, that's about it.

At some point, the character will become "Encumbered" and will move fairly slowly. At another point, the character will become so overloaded that movement becomes impossible. This is often the case if a mage hits you with a Ray of Enfeeblement.