You'll need a stipple brush for this. You can buy them in craft stores and they come in a variety of sizes. Think old shaving brush - the kind your dad probably used! Of course, they were huge, and you can certainly use them, but you'll mostly want something a bit smaller.
Choose two colours of ink. I used More Mustard & Cajun Craze.
Start by inking up your stamp in the lighter colour and then working quickly, dab your stipple brush into the darker ink. I kind of "swipe" mine across the ink pad. Then "stipple" (dab or tap) the brush onto the stamp in the areas you want to highlight. You may need to breathe on the stamp to moisten the ink before you stamp it onto your cardstock.
You can make the effect more dramatic by choosing different colours - Certainly Celery and Perfect Plum look amazing, particularly on large flower images.
After inking the stamp with Certainly Celery, the stipple brush was dabbed randomly all over the flower in Perfect Plum.
Or you can be more considered and stipple focal points like the centre of the flower and edges of the petals.
This would also be a great way to make leaves look like they're changing colour in the autumn.
TGIF! Hope the weekend is lots of fun and you can squeeze in some crafting.