There seems to be a big difference between the DOFF system on Tribble and Holodeck. My main character on TTS had approximately 60 Blue and 40 Purple DOFFs. (That's right, no white or green.) I had two additional characters and their ratios were close to this also. Every assignment ended with critical success. On Holodeck, I have 8 characters and none have Purple (except for the TTS bonus items). Only a few characters have one or two Blue. My VA on Holodeck has mainly white DOFFs with a handful of greens. I had a couple of blues but transferred them to another character. From my perspective though, rare is a lot more rare on Holodeck.
I mean I'm okay with that. Just pointing out the difference. The positive side of this is that my TTS bonus items are a lot more valuable to me. It also has me running the Colonization chains for those DOFF rewards. (I'm halfway through on Delta Volanis and Azlesa Expanse)
If you have a question about the DOFF system, post a note or send me an in-game message to @Rocketeer_7 I'll leave you with one tip: Don't waste your colonists on the Relocate Colonists To Federation Borders
assignment. Save them for the Colonial chain assignments.