When displaying two tubeplots together, we may specify their colors at will, as long as they are different colors! For instance, specifying red and green works correctly, but specifying red and red results in red and black!
See the attached worksheet. Interestingly, when displaying the contents of the worksheet on this website, the colors are rendered correctly! So don't go with what you see on this web page; look inside the worksheet instead.
PS: As a workaround, we may replace the red & red specification with
COLOR(RGB, 1, 0, 0) and
COLOR(RGB, 1, 0, 0.01)
which are different enough to make Maple happy, but produce essentially the same red color.