You can make any substitution that you choose. The difference between 3865 and B5200 is very slight. There is no rule that says you must use the suggested colors.

I love B5200 when I want the brightest white that DMC makes. 3865 looks a bit creamier, although it is nit a huge change from B5200. The regular "blanc" or white is somewhete between the two. I use B5200 when I want the white to really stand out, both from the fabricvand from the other colors used.

I also use B5200 when I want the white to really 'pop'. I like the white, to be white white at times, not just white, if you know what I mean.