which ceanothus has the most colourful and brightets flower

welcome to GoY.

They all have a similar flower form but the colour palet is from the palest blue through to a very deep blue.

perhaps you need to decide on its habit first. from dwarf through to dirty great big tree/shrub, upright/sprawling, slow/vigourous growth.

then you can research the varieties that fit your proposed planting space. in the back of my mind i have a feeling there might be a pink form but that could just be wishful thinking.