Does a chiral centre have to have 3 or 4 different groups attached?
The answer is that all chiral centers are stereocenters, but not all stereocenters are chiral centers thus the words are not interchangeable.

A chiral center requires four or more  different molecular groups attached to a single center atom (asymetrical). In the case of pnictogens (nitrogen, phosphorus, etc...) or chalcogens (sulfer, selenium, etc...) an electron pair can count as one of the molecular groups.

An alkene is a stereocenter that requires 3 or 4 different molecular groups such that no two common groups are on the same side of the bond (must be able to determine an E/Z configuration). In the case of the blue stereocenter, the methyl groups are on the same side and thus switching their position would create an identical compound meaning the alkene is not a stereocenter.  For the red center switching the position of the bromine and methyl groups would create the Z-isomer from the E-isomer thus is a stereocenter, but not a chiral center.  If however, the hydrogen of the blue stereogroup were to switch positions with a methyl group then the blue alkene would be a stereocenter with three distinct molecular groups.