Yes it is possible to color your dots differently based on a criteria. I don't know what your trend line looks like, so this may not fully solve your issue, this is how I would approach it:
The edit is done at the data set level, not at the chart level. Assuming you have data in 2 column format of X and Y axis, you need to split your Y column of data into two different Y columns, based on the criteria you have. Example: using an if formula, populate a Y_above and Y_below column using the formula of the trend line (compare y value against the output of the trend line function given the x value of the formula). With your new dataset of X and Y_above and Y_below (blanks in above where the value is below and vice versa), add the series to your chart and color them as needed.
