You can have a dynamic value on a textbox's text/fill colour using expressions.

Right click the appropriate cell and select text box properties. Navigate the the Fill Pane (I've assumed your doing the Fill colour, if you're doing text colour, then go to the font pane). Near the top, click the f_x button, which will open up an input window.

=iif([Insert logic here for when it's Green], "Green", "Red")

Seeing as you're trying to base this off data somewhere else, you may be better off actually returning the value of the colour you want in the dataset. You can even do this in the colours in Hexadecimal if you prefer. The expression you would use would then be simply (where FillColour is the column return with the colour you would like):

=Fields!FillColour.Value

Edit:

For sake of my sanity, is this what you would be expecting as a returned result set?

Create table #SSRSResults (ABC char(10),

Num Int)

Insert into #SSRSResults

Values ('A', 10),

('A', 12),

('B', 15),

('C', 20),

('B', 25),

('C', 30)

Create table #SQLTable (ABC char(10),

Num Int)

Insert into #SQLTable

Values ('A', 15),

('B', 15),

('C', 15)

Select R.ABC,

R.Num,

case when T.ABC IS NOT NULL then 'Green' else 'Red' end as FillColour

from #SSRSResults R

left join #SQLTable T on R.ABC = T.ABC and R.Num = T.Num

Drop Table #SSRSResults

Drop Table #SQLTable

This only returns B 15 in Green, the rest of Red.