From Reddit, take with a grain of salt.  But it's probably a good guideline.

Engram percentages:

Total engrams gathered 595:
Green: 290
Blue: 250
Purple: 55

Results:
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GREEN ENGRAM RARITY:
• White: 68 \ 23.45%
• Green: 201 \ 69.31%
• Blue: 21 \ 7.24%
• Purple: 0
• Yellow: 0
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BLUE ENGRAM RARITY:
• White: 0
• Green: 53 \ 21.2%
• Blue: 186 \ 74.4%
• Purple: 11 \ 4.4%
• Yellow: 0
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PURPLE ENGRAM RARITY:
• White: 0
• Green: 2 \ 3.64%
• Blue: 33 \ 60%
• Purple: 19 \ 34.55%
• Yellow: 1 \ 1.82%
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