As you can see on the shark below, the boarder between the white underside and blue/grey top side is a mix of both colours.
As has already been said feathering will be the way to go. This video
will explain the technique. I would paint the Caledor Sky colour first going slightly further up the model than where you want the boundary to be. Then paint your XV88 and Tau Light Ochre to the edge of the Caledor Sky. Using watered-down versions of the paints, alternate between the colours using feathering to create a blurry boundary. Assorted tips:
- Experiment with your technique on spare model bits or paper first.
- You may want much thinner versions of your darker colours than 2:1 as multiple applications of very thinned paint will let you slowly build up areas of colour
- Always paint in strokes towards the colour you are painting with as you will leave more pigment where you finish the stroke
N.B: The link takes you half way through as the first half is showing how to do it wrong i.e. blending by painting lines rather than feathering.