Why Sql Not Bringing Back Results Unless Set Varchar Size

I've got an SQL script that fetches results based on the colour passed to it, but unless I set the size of the variable defined as a varchar to (50) no results are returned.

If I use: like ''+@Colour+'%' then it works but I don't really want to use it in case it brings back results I don't need or want.

The column FieldValue has a type of Varchar(Max) (which can't be changed as this field can store different things). It is part of aspdotnetstorefront package so I can't really change the tables or field types.

This doesn't work:

declare @Col VarChar
set @Col = 'blu'
select * from dbo.MetaData as MD where MD.FieldValue = @Colour

But this does work:

declare @Col VarChar (50)
set @Col = 'blu'
select * from dbo.MetaData as MD where MD.FieldValue = @Colour

The code is used in the following context, but should work either way

<query name="Products" rowElementName="Variant">
<sql>
<![CDATA[
select * from dbo.MetaData as MD where MD.Colour = @Colour
]]>
</sql>
<queryparam paramname="@ProductID" paramtype="runtime" requestparamname="pID" sqlDataType="int" defvalue="0" validationpattern="^d{1,10}$" />
<queryparam paramname="@Colour" paramtype="runtime" requestparamname="pCol" sqlDataType="varchar" defvalue="" validationpattern=""/>
</query>

Also I can't set the size in <queryparam paramname="@Colour" paramtype="runtime" requestparamname="pCol" sqlDataType="varchar" defvalue="" validationpattern=""/>