[SalesForce] Change the font color of a lightning:formattedUrl

You can bind a "tabStyle" to the page, which causes the color theme/icon to match the named tab or object tab (Custom__Tab or CustomObject__c). However, this value cannot be dynamic, so you may need to use a template page or component that contains the logic you want to encapsulate for each object individually, then include that page in two other pages that have the desired tabStyle set.

Common.page:

<apex:page ...>
</apex:page>

Tabstyle1.page:

<apex:page tabStyle="objectA__c">
    <apex:include pageName="common"/>
</apex:page>

Tabstyle2.page:

<apex:page tabStyle="objectb__c"/>
    <apex:include pageName="common"/>
</apex:page>

This "trick" works because the CSS will be dynamically selected, and all subelements will take on the appropriate color attributes.

If the page itself does not need to be styled differently, but you want to change the style of individual page blocks, you can do this as well:

<apex:pageBlock title="Contacts" tabStyle="Contact"> ... This area is purple. </apex:pageBlock>
<apex:pageBlock title="Accounts" tabStyle="Account"> ... This area is blue. </apex:pageBlock>

Wrapping the apex:pageBlockSection in an element that a CSS class can be added to works:

<apex:page>

<style>
.custom1 {
    background-color: red !important;
}
.custom3 {
    background-color: green !important;
}
</style>

<apex:pageBlock>
    <apex:outputPanel layout="block" styleClass="custom1">
        <apex:pageBlockSection title="Section 1">
            Content 1
        </apex:pageBlockSection>
    </apex:outputPanel>
    <apex:outputPanel layout="block">
        <apex:pageBlockSection title="Section 2">
            Content 2
        </apex:pageBlockSection>
    </apex:outputPanel>
    <apex:outputPanel layout="block" styleClass="custom3">
        <apex:pageBlockSection title="Section 3">
            Content 3
        </apex:pageBlockSection>
    </apex:outputPanel>
</apex:pageBlock>

</apex:page>

producing this output:

PS

To style the headers you can make use of a CSS descendant selector:

<style>
.custom1 .pbSubheader {
    background-color: red !important;
}
.custom3 .pbSubheader {
    background-color: green !important;
}
</style>

where pbSubheader is a class applied by the Visualforce framework to the header.