Question
Is there any way to blend in the colours of our grass or will it do it automatically?

Answer
It depends! Do you have a pure stand or a mixture of grasses?
The color of the leaf of a plant can be a hereditary factor e.g. light green, dark green, variegated. We can change the color by fertilizer practices but as the effect of the fertilizer wears off, the coloration will return.
Sometimes, the color variation is due to a deficiency and it can be localised. The easiest example is a patch of lighter color observable on the lawns of newly constructed houses. This can often be traced to construction materials buried under the patch of turf.
So, go ahead and apply a granular fertilizer (16-4-8) which has slow release (more is better) nitrogen. But expect to pay more for the slow release property.