I'm assuming that the patch of (simulated) custard is on a separate layer.
If it is then a duplicate of the layer (as want to work as non-desructive as possible) and apply a displament map to it using the main trouser image as the displacement. This will make the contours of the folds in the trousers be applied to the custard layer, giving a more realistic look.
Then a case of applying opacity level and blend modes to the (displaced) custard layer.
This is just a short, tweak.(I could go really indepth), But hope it helps
Displacement maps look like they could be interesting for other things as well.
Your welcome
Yes Displacement maps can be very handy.
Another technique you may like to try when it comes a seeping effect, is to use the Blend If settings, by splitting the slider to 'This Layer' and 'Underlying Layer' you can get a good seeping effect, with dark patches etc. Again I hope it helps you.