Opal value is tricky most of the time, but fortunately colour is pretty straightforward.
It may surprise you to know that there is a lot of opal around, but almost none of it is valuable, and is therefore never mined and brought to the market.
The low value opal has no play of colour, which is the shiny colours that opal is well known for. It may surprise you to know that opal with no play of colour is almost identical chemically to opal with play of colour. Almost.
Opals get their colour from a unique arrangement of tiny little spheres that science goes here- if you are interested, I'll post and link an article. The way the particles occur means that reds in opal are the most rare and blues are the most common (relatively). This obviously has an effect on value.
The typical progression of colour from most valuable to least is therefore:
$$$$$ Red
$$$ Yellow
$$ Green
$ Blue
There are some exceptions, but this principal mostly holds true.