This is my Choko Nishiki in July.

That hosta is also called 'On Stage'. That is part of its "act" to change colors.

Here is what the library database says:
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H. montana 'On Stage' = H. 'Cho Ko Nishiki' - Japan
14" tall. Part Sun to Light Shade.
Yellow primary leaf color with a two tone green margin streaking to the center. Leaves 8 x 5. Purple flowers.
A large montana hybrid with yellow leaf, irregular green margins.
This is one of the most talked about hostas in any garden. The pointed, deeply veined leaves are green with a multi-tone gold center. It emerges very late in spring with a bright gold center that changes later to white, then to a light/dark green contrast late in the season. When planted in morning sun, it holds its color better through the summer. The 3' wide clumps are topped with light lavender flowers in early summer on 48" scapes.
Yellow leaves with irregular green margins, a rapid grower; lavender flowers.
The reversed form of montana 'Aureo-marginata'. Leaves are yellow with irregular two-tone green margins. One of the most beautiful new Hostas we have seen and certainly one of our favorites. Lavender flowers.
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-Babka

That's odd I don't remember either of my On Stages ever turning solid?! I will definitely have to pay more attention this summer! It says morning sun helps it hold it's color better maybe that's why mine don't change so much(3 hrs morning sun) and maybe that's why Anna's does? Anna is yours in a lot of sun?

Just when you think you know something.......
:)

AnnasHostas, I think the plant you have may be Fortunei Albopicta instead of Choko Nishiki/On Stage. Fortunei Albopicta is a viridescent hosta which means it turns green later in the season.

My On Stage/Choko Nishiki (I have two in different levels of sun) never turn green and don't have that leaf shape. They may turn a lighter shade of green later in the season but not anywhere close to the dark green of your hosta.

Vanessa

Are you sure that's Choko Nishiki? Doesn't look like the Library pics and the database says nothing about it turning green. My On Stage doesn't turn green at all.

Steve

I agree that first one pictured is not Cho Ko Nishiki or On Stage. I just saw the name and remembered that it put on quite a show. I don't have f.Albopicta, but when I go to the Library to look, sure enough, the one in the OP is a dead ringer.

-Babka

Steve- It describes it turning to a light/dark green contrast late in the season. I copied and pasted that description from the db.

-Babka

Babka's got it right. That plant is Fortunei Albopicta.

Steve

I'd love to take credit for the ID, but I messed up. Credit goes to Hippieindenile!

-Babka

Thanks everyone for your input. I am (or was) quite sure that it is Choko Nishiki. I bought it from one of the most reputable suppliers in UK, Bowden Hostas (www.bowdenhostas.com).
I doubt they would make a mistake, but then anything is possible.
Another supplier: Mickfield Hostas (http://www.mickfieldhostas.co.uk/varieties/0666.htm) does actually show the leaves turning greener in the summer, but not as green as mine. Please see close up pic.
Perhaps alyciaadamo is right, not enough sun in the morning. This year it is in full sun (if and when we get some!). Otherwise, you could all be right and it is Fortunei Albopicta.

This Forum is brilliant! I am so pleased I've found it!
Thanks again.
Anna