Very happy with Beyerdynamic DT990 open-backs and a good headphone amp. I use the 250 Ohms. I also have a pair of DT770s for closed-back work that are 80 ohms and those work well with a wide variety of amps, though not quite as "clear" as the 990s, I'd not call them coloured, but "denser". The 990's are quite open.
None of these are the same as a pair of Audeze or something, but for the money go audition some Beyerdynamics and the Sennheiser 6xx and 8xx series, and some of the higher end AudioTechnicas if you care to. I found the Beyers suited my listening preferences and translate accurately (I've done some headphone-only mixes on the 990s that were quite well received), but it's going to be up to how you hear and how well you can translate - any decent pair of 'phones that let you hear everything in the sound will be workable to mix on - you learn to compensate for any differences like you do with speakers in a room, so it needn't be exact. Stuff like Bose and consumer / pop / noise cancelling headphones you should stay the heck away from as they introduce nasty phase distortions and overemphasized eq curves mask other sounds.
As for the Sony MDR7506 - these are a classic recording pair, as they're very pronounced in the midrange and are perfect for auditioning the vocal range well. Unfortunately they suffer from a lack of good ear sealing on many people's heads (mine, certainly) and are rather lacking in low end extension/clarity, so I can't recommend them for mixing, but ymmv. I have a pair, in fact they were the first pair of decent headphones I bought myself decades ago, and I still prefer them for camera work as the vocal range pops out of the background very well on them - they're great broadcaster cans too, for that same reason.