Hi Wolfgang,

Thank you for your reply. I did as you suggested but it still never 
worked. However it did make me delve further into the Metafun, 
LuaMetaFun and the Colouring ConTeXt manuals where I found examples of 
the *resolvedcolor* helper which I used as follows:

definecolor [name = "myColor", r = uniformdeviate(1), g = 
uniformdeviate(1), b = uniformdeviate(1)];

for loop..

fill unitsquare xscaled 5cm yscaled .5cm shifted (7cm,i*cm) 
withcolor(r[resolvedcolor("myColor"), white]);

endfor;

I also found the *namedcolor* helper and I could use it as follows:

fill unitsquare xscaled 5cm yscaled .5cm shifted (1cm,(2*i)*cm) 
withcolor namedcolor("myColor2") ;

However, for some reason it did not work in:

fill unitsquare xscaled 5cm yscaled .5cm shifted (7cm,i*cm) 
withcolor(r[namedcolor("myColor"), white]);

//

Thanks again for your suggestion//

Best Wishes

Keith//

On 28/04/2022 16:04, Wolfgang Schuster wrote:
> Keith McKay via ntg-context schrieb am 28.04.2022 um 12:05:
>>
>> Hi,
>>
>> In the MWE below I define an rgb colour, myColor1, using random 
>> numbers for the r, g and b components. I then use it to fill a 
>> unitsquare the size of the page, which works as expected. In the loop 
>> that follows I then use the defined colour, myColor1, along with 
>> white and the variable r to create shades of myColor1 as in the MWE 
>> below. However, myColor1 is not recognised in the loop and seems to 
>> be replaced by black, although if I use a predefined colour, in this 
>> case BurntSienna from the crayola collection, it works as expected.  
>> I have tried a number of ways to get this to work as shown in the 
>> code but without success. I would be grateful for any suggestions.
>>
>> Best Wishes
>>
>> Keith McKay
>>
>> %%%%%%%%%%%%%%%%%%%%MWE%%%%%%%%%%%%%
>>
>> \setuppapersize[A4,portrait]
>>
>> \usecolors[crayola]
>>
>> \starttext
>>
>> \dorecurse{10}{
>>
>> \startMPpage
>>
>> StartPage;
>>
>> \definecolor[name="myColor1", r = uniformdeviate(1), g = 
>> uniformdeviate(1), b = uniformdeviate(1) ] ;
>>
>
> Remove the backslash in front of the definecolor command.
>
> Wolfgang
>