I have a small sketchbook I take on my daily walks. I normally take a marker with me. However after noticing these flowers I decided to take a Preppy pen filled with watersoluble ink.
The Platinum Preppy pen is an inexpensive fountain pen costing around $4.50. Yes very affordable. It comes with a cartridge for which can buy refills. You could also slip a Platinum converter into the pen. This would allow you to use any ink. However, the converter would cost more than the pen at approximately $8.00.
So another alternative is to remove the cartridge, wash it out (Murphy’s Oil Soap) and refill it with a syringe. The syringe costs 99 cents in the animal section of the local farm store. The ink is a sample from Goulet Pens for $1.00 . Both Goulet pens and Jetpens sell the pen. However, I believe only Goulet pens has the ink samples. The sample of ink in the drawing is Noodlers Antietam ink. It is really beautiful! Made for Fall!
Here is what I used and the links to Goulet Pens and Jetpens. It looks like the pen ranges in price from $3.00 to $4.45 depending on the size of the nib. I believe I added some Transparent Orange watercolor. However, just a little. You are looking at the color of the ink.