Perceived difficulty of dual position 2-back is likely to be largely
dependant on the strength of your auditory n-back skills imo.
My experience with DNB is outlined below:
I can play dual 7-back (though I usually play D6B) only because I
score so high on position-match trials that it compensates for a
failing score on the auditory-match trials -- matter of fact, I can't
consistently pass anything beyond 5-back on the auditory side of
things, yet extremely high scores on the position-match trials (often
100% and rarely much below 90%) often lead to D5B scores in the 80-90%
range, leading me to play D6B where I promptly begin failing on the
auditory-match side of things, although scores still seem quite
respectable when you score ~ 80% on the position side and 40% on the
sound side...
As for dual position n-back, I typically score ~ 70% on dual position
3-back with scores most often in the 60%-80% range, rarely exceeding
85% and very rarely ever failing (the availability heuristic is
probably at play with my remembering of these extreme events).
I thought it was worth mentioning my experience granted I am not a
whole lot better than you at D5B (and certainly not much [if at all]
better on the auditory side of things) and yet I find it rather easy
to score 90%+ on dual position 2-back.
That you say ''Color is especially hard to memorize'' seems to me to
indicate you're probably having trouble for the same reason I do if I
try to play dual position 3+ back (eg. I start mixing up or completely
forgetting whether a certain position element was blue or green but
can very well remember [irregardless of whether I play intuitively or
try to memorize which colors were where] that the top-left had a
position match for 'some color' n-trials ago). It would be interesting
to see if those who have trained with triple n-back (or another mode
which also trains memory for color) have an easier time with dual
position n-back. I'm not sure how long you have been playing dual
position n-back -- I had a lot of trouble with 2-back myself when I
started using it. I went for some time playing dual position 3-back
and, at the very least, I believe it helps maintain any gains on
position n-back resultant of dual position n-back training -- I went
for a few months without almost playing DNB or position n-back
whatsoever and dual stim position 3-back led to no loss whatsoever in
scores (or perceived increase in difficulty) when I returned to D5B/
D6B and position 6/7-back earlier this year.