I'm glad to hear you solve the problem and thanks for your sharing!
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others.". So, I would make a brief summary of this thread.
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community members to see the useful information when reading this thread. Thanks for your understanding!
[SharePoint List conditional formatting MultiChoice field]
Issue Symptom:
OP created a SharePoint list with a multi-choice field called "Equipo" that can take several values at the same time. OP have tried the following:
"background-color": "=if([$Equipo]=='FA','#FFF3D6',if([$Equipo]=='GC','#DEF7EB','#E7E7EE'))"
It worked when "Equipo" was a Choice field. However, OP needed this field to be multi-choice, and now it is not working. OP would like that if Equipo has one option, card looks with a particular color. Otherwise (several options selected), in white.
Current status:
OP found the solution.
"background-color": "=if(length([$Equipo])>1,'#EFF1F4',if(indexOf(join([$Equipo],''),'Team1') != -1,'#EFFBF0',if(indexOf(join([$Equipo],''),'Team2') != -1,'#EEF5FC',if(indexOf(join([$Equipo],''),'Team3') != -1,'#F2EFF7',if(indexOf(join([$Equipo],''),'Team4') != -1,'#FEEEEC',if(indexOf(join([$Equipo],''),'Team5') != -1,'#FFF9EB',if(indexOf(join([$Equipo],''),'Team6') != -1,'#F0FAF9',if(indexOf(join([$Equipo],''),'Team6') != -1,'#F5F5F5','#EFF1F4'))))))))"