Performance impact of including result column in indexed columns
Based on the information given, I will take a shot at an answer...

You stated


  If I add identifiedtex as the last column indexed in the index, will this speed up a lookup of the texture since the result is part of the index?


Let me say that you have the following query:

SELECT identifiedtex FROM texels
WHERE texelx = 210938
AND texely = 378432
AND texelhue = 23;


What would be the effect of having an index (call it Index3)

texelx, texely, texelhue


versus this index (call it Index4)

texelx, texely, texelhue, identifiedtex 


When you retrieve identifiedtex using only Index3, it will require an additional table lookup to ascertain the identifiedtex from the table.

When you retrieve identifiedtex using only Index4, it will an index-only lookup because all mentioned columns are in the index. It is known as a covering index.

I mentioned covering indexes in my past posts:


Mar 12, 2012 : How to index this table (a_level, b_level, item_id)
Oct 17, 2012 : Combining columns in index
Nov 13, 2012 : Must an index cover all selected columns for it to be used for ORDER BY? (See My Conclusion)


So to answer your original question, yes adding identifiedtex will index make for a faster query.