mine has had some red leaves and I put it down to high temps and a lack of water. But there may be a better explanation. They have greened up this spring.

Welcome to GoY too :o)

They look chlorotic to me, Samparis. That is usually caused by alkaline or salty soil or water. A soil test may be in order, and sometimes the local Cooperative Extension can do that for you. Otherwise, there are many private labs that do soil and water tests.
If the soil pH is too high (>7.5), the solution is usually deeper, less frequent watering, switching out the rock mulch for an organic one, and/or applying soil sulfur pastilles--rather than the crystalline form.

Thank you for your comments. They are very helpful. Our soil is Ph 6.5 according to my tester.

Hmm...6.5 would be just about ideal for Oregon Grape. Maybe some rare mineral deficiency, like boron, or copper? Also, does your tester reach at least 6" underground? Have you ar a neighbor used a weed killer such as prometon or glyphosate nearby in the last two years? Adding a close-up of the newest leaves would tell us more.

that could explain mine then Tug as my soil in that area is pH 7.5 over bed rock chalk. As it is not far from a Camellia that I do give a feed designed for them [to prevent chlorosis] that would help the Mahonia stay green.

Severe lack of nitrogen could cause those symptoms, too. If it hasn't been fed recently, a handful or two of blood meal might help.