The above oil painting is my latest and has quite a bit of blending of colours to it. I tend to start with the distance and work my way to the foreground. In this case, the sky was first. There are several colours that are blended. Red, orange, yellow, cerulean blue, cobalt blue, magenta, a touch of Payne's grey and as always, lots of white.
Once I roughly know where I want the colour, I lay each colour down with a large brush for each colour. (75 mm or larger, depending on the canvas size) I work quite large. It's important to have a separate brush for each band of colour so they don't mix together.
The paint is mixed with some No: 1 Medium (linseed and varnish mixed together, also helps to dry the paint quicker) then applied in a band of that colour starting at the lightest tone (horizon) and then moving up to the darker tones at the top of the painting. Using lights and darks in this way gives the sense of depth and distance. The further away from you, the lighter is the tone and the closer to you the darker it becomes.
Don’t use turpentine to mix with your paint as it is a paint thinner and you will lose the rich pigment colour in the paint. Turps is only for cleaning your brushes at the end of a session or for applying a thin wash as a preparation. (I use odourless turps solvent, then there are no issues with the smell or headaches) Be careful that each colour doesn't overlap at this stage as it will change the colour. For example, if the blue and yellow overlap it will turn green. There will be a slight change of colour at the join but that’s OK, it's a natural look. For this reason, it is good to get to know your colour wheel. Complimentary colours, mixing of colours etc
Once you have the bands of colour down and they are still wet, then comes the fun bit. Get a large dry brush and starting at the lighter area, swipe the brush back and forward in a crisscross method over where the two colours meet. This will gently blend them together. Clean and dry the brush and go up to the next two bands of colour and do the same. Continue this on until all the bands are blended. Step back and look. If it still looks in bands then do more dry blending.
So this is how I get my delicate blending of sky and water in my oil paintings. This method is something that requires practice so don't be disheartened if at first, you don't get it right. It's like anything you want to improve in, you have to keep at it.