PR101 is synthetic red iron oxide. Depending on how the pigment is treated or heated, the resulting watercolour paint that's created from it can be transparent or opaque, appear reddish or brown. It's a versatile pigment for making colours and Daniel Smith has eight colours made from this pigment.
Venetian Red is an extremely opaque warm red. The Italian version is made with pigmented mined from Italy.
Transparent Red Oxide and Transparent Brown Oxide can produce really beautiful granulation when mixed with blues. Transparent Red Oxide can be considered a more exciting version of Burnt Sienna. Transparent Brown Oxide can be the alternative to Burnt Umber.
Right is English Red Ochre with French Ultramarine. This mix is kinda similar to the Burnt Sienna-Ultramarine mix.
And these are mixes with Transparent Brown Oxide, Transparent Red Oxide with French Ultramarine. The mix on the right is visually more interesting because you can see red in blue when the colours aren't mixed completely.
All these paint can be found on Amazon (links above) and Jackson's Art UK