Ice the cake with BC in the lightest background color, it looks like a cream or very light yellow to me. Then do thick fondant stripes in the darker yellow and thin stripes in the orange-ish color. You should be able to just brush a little bit of water on the backs of the stripes before applying to the cake to make them stick.
i vote for the easy way out...
paint on fondant.
just use two different size brushes -- one a 1 or 2 inch wide foam brush (washed and dried in advance)
and then a small 1/4" or so FLAT artist brush.
basically it's the same yellow -- just at two different dilutions. -- very thinned out for large stripes and more concentrated for the darker lines.
painting on fondant is much easier than it sounds and... it could give you that water color effect too
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