Removing null values from array object mongodb

Option 1: using aggregation pipeline inside update() query with $filter

db.collection.update({
 color: {
  $exists: true,
  $eq: null
 }
},
[
 {
  $addFields: {
   color: {
    $filter: {
      input: "$color",
      as: "c",
      cond: {
        $ne: [
          "$$c",
          null
        ]
      }
    }
   }
  }
 }
],
{
 multi: true
})

Explanation:

1. Match only documents where array color exist and contain at least single null value
2. Replace the color array with same with no null values
3. Add {multi:true} to apply to all found documents

playground

Option 2: remove all null elements in color array with $pull

db.collection.update({
 color: {
   $exists: true,
   $eq: null
 }
},
{
 $pull: {
   color: null
}
},
{
  multi: true
})

Explanation:

1. Match all documents having color array and at least single null element.
2. Remove ($pull) all null color array elements
3. Apply to all matched documents (multi:true)

playground

I would use the option 2 since it looks simple, but in certain cases for bigger collections, the $pull operation may perform slower so is better to test ...