All chillies start off as green. As they ripen, they turn red or yellow.
Also the riper and redder - the sweeter the chilli will be.
Red chillies are usually a bit milder but more pungent. 
Green chillies are usually considered a bit hotter.
Rule of thumb is the smaller the chilli the hotter it will be.

Select chillies with a firm skin that is rich in colour.

WA - Outer Metropolitan Areas, South West, Canarvon.