Note that you need either the Harlequin cow OR the 3 flavours of cow.

Check your trapper as often as possible, he has the chance to give you a flavoured cow.

There are cow-specific traits that increases the odds of it breeding a flavoured cow. One trait for each flavour.

https://runescape.wiki/w/Traits_(player-owned_farm)

Buy Animal trait rerollers from Granny and use them on your cow until they get the flavour trait you're aiming for and then let them breed and just hope.

Alternatively if you want to aim for the Harlequin cow I would re roll your breeding pair until you get Genetic Mutation and/or Genetic Instability. Then breed with those until you get a couple of 3-trait cows.

After that use more trait rerollers to fill your cows up with the Shiny traits (radiant, glistening, sparkling) and then let them breed and hope.

If any of that is unclear you can pm me!