The colors of the example it's not something to worry about.
Looks like that they are playing with the percent of Red and Green.
About the position and the value.
Let's say that chooseble area it's from X1 = 10 , Y1 = 10 to X2 = 60 and Y2 = 40. You will have to check if the given EventX() and EventY() are inside this box and then to find the position of the cursor and the value.
Value = ( (EventX() - X1) / (X2 - X1) ) * 10
Beware to use floats.
Looks like that they are playing with the percent of Red and Green.
About the position and the value.
Let's say that chooseble area it's from X1 = 10 , Y1 = 10 to X2 = 60 and Y2 = 40. You will have to check if the given EventX() and EventY() are inside this box and then to find the position of the cursor and the value.
Value = ( (EventX() - X1) / (X2 - X1) ) * 10
Beware to use floats.