The only stat that works with legendaries when it comes to HDs (at least for me) is health. Which is easy to check.
My chances with HD6 with Army's 1% HP are on orange. I was healing my Army in multiple steps (2k HP per check) until I reached yellow indicator of chance. After that I started to de-equip one of the legendaries, Ashmodai. I took off his 2975 points-worth legendary weapon and I WAS STILL AT YELLOW, so if it really counted ATK and DEF I would've slipped back to orange. Then I took off another legendary item, the armor slot one, which is worth 1925 total points. I WAS STILL AT YELLOW. And only after I took off the third item, I slipped back to orange. Coincidentally (not really), the HP part of those 3 items was worth 2125 points, which falls off that last 2k HP healing step. Meaning my legendaries for god-knows-what reason benefit only from HP but get no points from ATK or DEF. And it doesn't matter what kind of equipment you put on the legendary, he only benefits from HP.
This might also be the reason why you got a switch to the next colour of chance when you summoned Luperas, you simply got enough HP bonus points to do so, but the ATK and DEF didn't count.
Please check the mechanics behind the ATK and DEF stats, and also give an extra check to the legendary items themselves. Taking off gear is the easiest way to check the stuff. I was taking off Ashmodai specifically, didn't check with other legendaries but something tells me it's an overall issue.
Hoping to hear from you...
