Cambiar color <p:autocomplete>

Agregar al css

.ui-autocomplete {
    background-color: red;
}
.ui-autocomplete .inputTest  {
    background-color: red;
}
.panelTest .ui-autocomplete-items .ui-state-highlight {
    background-color: blue;
}
.ui-autocomplete-multiple-container{
    background-color: red;
}

Fuente de css de autocomplete

https://jar-download.com/artifacts/org.primefaces/primefaces/6.0/source-code/META-INF/resources/primefaces/autocomplete/autocomplete.css

Agregar los css

 inputStyleClass="inputTest" panelStyleClass="panelTest"

 <p:autoComplete dropdown="false"
                                 multiple="true"
                                                                         scrollHeight="250"
                                        size="15"
                                        emptyMessage="#{app['info.nohayregistros']}"
                                        value="#{solicitudController.facultadList}"
                                        completeMethod="#{solicitudController.completeFiltradoFacultad}"
                                        var="p"
                                        required="true"
                                        inputStyleClass="inputTest" panelStyleClass="panelTest"
                                        itemLabel="#{p.descripcion}"
                                        itemValue="#{p}" forceSelection="true">
                            <f:converter binding="#{facultadConverter}"/>
                            <f:attribute name="field" value="descripcion"/>
                            <f:attribute name="fielddropdown" value="false"/>
                            <f:attribute name="fieldquerylenth" value="1"/>
                            <p:ajax event="itemSelect" listener="#{solictudController.handleSelect}"
                                    />
                            <f:facet name="itemtip">
                                <h:panelGrid columns="1" cellpadding="5">
                                    <h:outputText value="#{msg['field.descripcion']} #{p.descripcion}" />


                                </h:panelGrid>
                            </f:facet>

                        </p:autoComplete>