How to colour space between flagstone?

You could add colored gravel between the flag stone, add mulch around them or even sand. We used polymeric sand to give a firm seal into pavers. This hardens and makes a unfriendly place for weeds to take a foot hold.

If you have some time and patience grow grass or another type of cover around the pavers but that depends on what you have in your yard.

Could you plant some sort of ground cover flowering plant?

You can color sand with food coloring and tamp down.

I planted creeping jenny it is such an easy plant to care for and is a very pretty green it spreads and really fills in the spots

you could also use pea pebbles they come in white and multi colors and it looks nice too and easy to do

Hello. It depends on the type of material you wish to use.. Colored crushed rock or a plant might be considered.

I was thinking along the lines of colouring polymeric sand with the powder used in cement or creeping thyme in pink or blue

If the area already has mortar, you can stain or paint it.

If it isn't, fill with colored sand or rock or colored mortar.

thank you so much

What type of material is in the space? Grout? If so then you can get colored grout and then seal it when done

Most people use polymeric sand and it comes in colors. Some very basic colors are available at Home Depot, like gray or brown, but a masonry supply store will have a wider choice.

Is this for a patio

can you show pictures what you mean ?

You can add grasses in-between instead of paint.