There are two ways how i approached army painting a red army.
The lazy way is to undercoat your model in chaos black and then get a red from the Army Painter Coloured sprays and use directional spraying lightly to build up a gradient and it solves the issue of shading parts of the model. Then tidy up the model with mechrite, blood red and add bleached bone to the red for final highlights, using badab black or the brown wash (don't remember the name) to darkline the features of the model.
The longer tedious way is to use a black undercoat and add scab red to the chaos black and work your way up to blood red then bleached bone again for highlights. Bleached bone is used as it gives a much warmer light red whereas white will just give you a strawberry milkshake pink.