Update v2 oData model with linked property failed

I developing a masterdata SAP ui5 using an odata v2 model. The model looks as follows: A CAR object with an attribute COLOR as reference to a table colors:

    <EntityType Name="Car">
<PropertyRef Name="Id"/>
<Property Name="Description" Type="Edm.String" Nullable="true" MaxLength="100"/>
<Property Name="Color" Type="Edm.Int32" Nullable="false"/>
<Property Name="Id" Type="Edm.Int32" Nullable="false"/>
<NavigationProperty Name="ColorDetails" Relationship="testApplication.Car_Color_Many_ZeroToOne0" FromRole="Car" ToRole="Color"/>

<Association Name="Car_Color_Many_ZeroToOne0">
<End Type="testApplication.Car" Multiplicity="*" Role="Car"/>
<End Type="testApplication.Color" Multiplicity="0..1" Role="Color"/>
<Principal Role="Color">
<PropertyRef Name="Description"/>
<Dependent Role="Car">
<PropertyRef Name="Color"/>

This data is bound in a SAP ui5 simpleForm to input fields and the color to a combo box, listing all available colors so that the user can select a color:

<ComboBox selectedKey="{Color}" selectionChange=".validator.validateComboboxSelected" items="{/Colors}">
            <core:ListItem key="{Id}" text="{Description}"/>

The update in the backend (oModel.submitChanges()) fails with the error "Request body is malformed."

What is wrong ?