(Late to the game, answering for the benefit of future visitors to the SE).
In this case you should use neither a multivariate time series model, nor a univariate model with causal variables. What you need is a hierarchical model for grouped time series.
In your case, you would forecast each product separately, and then group the products into a hierarchy of products: Red apples and green apples fall into a group called apples, blue berries and strawberries into a group called berries, etc..and then put apples and berries into a group called fruit, etc...the you can reconcile the forecasts at different levels using top down or bottom up approaches.
How you group your series will be based on domain knowledge and business considerations.