<ui:inputCheckbox aura:id="setColor" label="Set Color" change="{!c.onSetColor}" />

<ui:inputSelect aura:id="colors" label = "Colors" class=""/>

onSetColor: function(component, event, helper) {
        var checkCmp = component.find("setColor").get("v.value");
        console.log(checkCmp);
        if(checkCmp){
            component.find("colors").set("v.value", "Red");
        }
        else{
            component.find("colors").set("v.value", "Blue");
        }
    }

<ui:inputCheckbox aura:id="setColor" label="Set Color" change="{!c.onSetColor}" />

    <aura:attribute name="colors" type="String[]" default="Red,Green,Blue"/>
    <lightning:select aura:id="select" name="select1" label="Select a Color" required="true">
        <aura:iteration items="{!v.colors}" var="color">
                <option text="{!color}"></option>
            </aura:iteration>
    </lightning:select>

({
    onSetColor: function(component, event, helper) {
        var checkCmp = component.find("setColor").get("v.value");
        if(checkCmp){
            component.find("select").set("v.value", "Green");
        }
        else{
            component.find("select").set("v.value", "Blue");
        }
    }
})

<aura:attribute name="contactLevel" type="String[]" default="Primary Contact,Secondary Contact,Other"/>
    <ui:inputSelect aura:id="levels" label="Contact Levels" change="{!c.onSelectChange}">
        <aura:iteration items="{!v.contactLevel}" var="level">
             <ui:inputSelectOption text="{!level}" label="{!level}"/>
        </aura:iteration>
    </ui:inputSelect>

({
    onSetColor: function(component, event, helper) {
        var checkCmp = component.find("setColor").get("v.value");
        if(checkCmp){
            component.find("levels").set("v.value", "Secondary Contact");
        }
        else{
            component.find("levels").set("v.value", "Primary Contact");
        }
    }
})