Partial colour change of a line

Hey,

I am desperate!

I want to change the colour of a line but partially.How I can do this?

I use a laser mashine CO2  and by changing the colours of a design you can deside either you want to cut or engrave.

I have a blue line for example and I want to change the colour in a part of that line and make it red.It seems easy but I cannot do it

Please help me!

Thanks!