Google contacts background color change

From Dec 2017, I am using my new Android device attached with my old Google id. The contacts got automatically synced. And, works without any issues. But, today I have noticed that, one of the contact's background(default color added automatically) color got changed automatically. I haven't edited contact.(Sometimes editing contact changes color). For Example: Contact A: contact's background is black previously, now changed to Pink. Contact B: contact's background is green previously, now changed to Dark red.(N.B: Contact A and B have the most number of call logs in my phone) Why's this happening like this? What's the logic behind it? If it is regular/normal, does this happen in any interval? I haven't noticed this on previous device.