Hi Hema,

Keep in mind that these ROIs are datasets of small integers,
and that via '-expr a+b' you are adding those numbers.

It seems likely that you may not even want an a+b operation.

Suppose that you have one dataset with 3 ROIs (values 0-3),
and another with 5 (values 0-5). When you add them you
will have a dataset with values between 0 and 8 (depending
on how they overlap). If at some particular voxel there is a
2 in the first dataset and a 3 in the second, that voxel will
have a 5 in the output.

So what would a value of 5 mean? It could be a combination
of 0 in the first and 5 in the second, or 1 in the first and
4 in the second, or 2 in the first and 3 in the second, or 3
in the first and 2 in the second. Since 3 is the biggest
number in the first dataset, there are only those 4 ways to
get a value of 5 in the 'sum' dataset.

But this would most likely be a poor choice for how to put
these ROI datasets together, since the output values will
be ambiguous.

---

Separate from that is the question of what their colors are.
Just keep in mind that the color is given by how the range
of values maps to the color bar. Zero is either blue or
green (depending on whether you use 'Pos'), and the maximum
is red (for the default color bar). The other values are in
between.

Note that you can right-click on the color bar and see which
value that color corresponds to.

Does this help?

- rick