From my limited experience with MDF doors, door grade MDF panels tend to look a bit yellowish and those that are not suitable for door machining tend to be on the brownish side. Maybe it has something to do which type of wood is used to make it from. But I'm not sure it's a general rule.
Anyway, you need to google it for "door grade MDF", or "moulding grade MDF", "MDF double refined",... or something similar. I'm not sure there's a standard name for this things, looks like every MDF manufacturer has his own idea about the name, or you could ask your local suppliers for more details.
Two of my local sheet suppliers have no idea what I'm talking about when I ask about that, but the third one carries those boards, made by McEwen and the description of the product says this:
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MDF, Double Refined (Door Grade)
Door grade MDF is designed specifically for painted and bladder pressed doors for the cabinet door industry. Quality standards on this product ensure better machineability with no raised fibers. This is achieved by a more uniform density throughout the board with a flatter profile. 1/2″ – 49×97, 61×97 5/8″ – 49×97, 61×97 3/4″ – 49×97, 61×97
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Some time ago Home Depot in my area used to carry those yellowish MDF boards that machined pretty good (smooth finish, no fibers) and they were also cheap. I don't know what happened but they don't have them any more.