Paul, it's rarely a good idea to "dumb down" requirements as generally speaking the complexities dictate the viable approaches
Also, sample files are more helpful than images (for all concerned)
Using the basic example a SUMIF would suffice - given we can't see range references we will have to make them up:
In the above we are assuming the lists are linked to cells C2:C3 and that colours appear in B11:B16 and sizes C10:D10
You could also use an INDEX/MATCH but SUMIF has added advantage that it will return 0 by default should invalid selections be made.
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