A few of you will have run in to this issue, in Project Web App both in Project Server 2013 and Project Online. In some drop downs we have a transparent background which makes it difficult to read the list of choices as the text behind is also visible, The example below is choosing a custom field at the task level – and the custom field is based on a lookup table. You will also see similar behavior in other areas of the product – such as using the force in/out option in the Portfolio feature. For those of you using Project Server 2013 on-premises you have the answer – the fix was shipped in the April 2013 Cumulative Update – see the second screen shot. For the users of Project Online this same fix will be deployed over the next few months (June-July timeframe) – so you will see the problem go away soon.
Here is a view of the same thing in my Project Server 2013 installation which is patched to the April 2013 Cumulative Update
One other observation while I was testing this scenario – you do now see the default values set for custom fields applied when you create a new task within PWA – and also at the Project level when creating a new Project via Project Center (although it isn’t shown in the PDP while you are creating – it will be there once the project is created).