I highly disaggree with any advice that includes the words "sell all grey mods". Just no. I'm constantly hitting the cap, so I feel your pain.
However, grey mods all need to be leveled up to 12 to see if they have speed of 4 or 5. If they don't, most likely sell it. But if it does have 4 or 5 speed, level to 15 and slice it so see what hits. If speed hits, keep slicing. If speed does not hit, then most likely sell it.
Green mods to 9 and same process, only you will know when you hit level 12 if you should sell. Blue to 6 and Purple to 3.
But never "sell all your grey mods" ever.